Software Development
13 minutes reading time
An analysis of multiple projects found that, on average, 18.5% of code lines were duplicates. That means nearly one-fifth of your codebase could be making your job harder instead of easier.
You might not notice it at first, but code duplication slows development, clutters your workflow, and increases the risk of bugs. Every extra block of code you maintain adds time and effort to debugging, testing, and refactoring.
In this article, you’ll understand why this is a problem, how to avoid code duplication, and what steps you can take to keep a maintainable codebase. So, let's dive in.
What Is Code Duplication?
Code duplication happens when the same code fragments appear multiple times in your code base. This might seem harmless at first, but as your project grows, these instances of duplication become harder to track and maintain.
A study found that duplication can be significant in visual programming environments, with some codebases exhibiting up to 39% duplicated code. The more entities and features you introduce, the more duplication spreads.
You may start with a simple function but later realize that similar logic appears in different places. Over time, this leads to unnecessary code complexity, which makes debugging, testing, and duplicate code refactoring more difficult. Writing clean code means recognizing and addressing duplication before it becomes a problem.
What Is an Example of Duplicated Code?
Think about tracking employee work hours in your application. You might write a function that calculates the total hours worked. Later, you need the same logic to generate payroll reports and another to log time for clients.
Instead of reusing the function, you copy and paste it into both places. This code repetition leads to multiple identical fragments, which increases maintenance efforts.
If you need to update how hours are calculated, you must modify the function in multiple locations, increasing the risk of errors. Recognizing and fixing duplicative code early improves maintainability, reduces technical debt, and helps you scale efficiently.
Causes of Code Duplication
As your code base grows, duplication can sneak in, making maintenance more difficult. Sometimes, it happens by accident. Other times, it’s a shortcut to meet deadlines. Whatever the reason, letting code duplicate too frequently leads to unnecessary complexity and complicates your job in the long run.
Here are the most common causes of duplicate code:
- Copy-paste coding: Instead of refactoring, you might copy and paste a function into another class to reuse it quickly.
- Independent development: Multiple developers might unknowingly write similar logic in different places, creating identical code across your project's sections.
- Poor architecture: Without a clear structure and following SOLID principles or primitive obsession, code becomes scattered, making it hard to identify reusable logic. A lack of coding standards leads to code smells that encourage duplication.
- Time pressure: When deadlines are tight, you may copy existing logic instead of extracting reusable functions because you might think you’ll clean it up later (but typically don’t).
- Subtle duplication: Some duplication isn’t apparent. Two code sequences may look different but perform the same task, which leads to hidden redundancy.
- Unreadable code: If the original implementation is hard to follow, you might rewrite similar logic instead of reusing existing code.
In typical programming languages, focusing on code reusability reduces maintenance efforts and prevents unnecessary duplication. Writing generic code without considering long-term scalability can increase costs. This makes code clarity and structure essential for efficient code in software development.
Is Duplicate Code Good or Bad?
Duplicate code is generally bad, but not always. Some duplication makes your codebase harder to maintain, while other cases are necessary to keep the code clear.
If you try to remove all duplication patterns too soon, you risk introducing a wrong abstraction that makes future updates more difficult. The key is knowing when to refactor and when to leave it alone.
Also, removing duplication without understanding its purpose might make the actual code harder to work with, increasing complexity instead of reducing it. The lousy code costs add up over time, slow progress, and create unnecessary obstacles in code during development.
When Duplicate Code Is Bad
In most cases, duplicate code slows development. It forces you to make the same change in multiple places, which leads to inconsistencies and errors. Here’s when duplication causes problems:
- It decreases maintainability: Every update requires changes in multiple locations, which increases the risk of missing something.
- It slows development: You waste time searching for and updating duplicate code instead of working efficiently.
- It increases the risk of bugs: If you update one instance but forget another, the system may behave unpredictably.
- It leads to technical debt: Over time, unnecessary duplication clutters your code base and makes it harder to refactor.
- It signals poor architecture: When similar logic is written multiple times, it typically indicates that the system needs a better structure.
Good Code Duplication: Essential vs. Accidental Duplication
Not all duplication is bad. Marius Bongarts challenges the conventional DRY or "Don't Repeat Yourself" principle by emphasizing that not all duplication is detrimental. He highlights the difference between essential and accidental duplication.
Essential duplication happens when similar logic appears in different parts of your system for valid reasons. Accidental duplication occurs when the same logic is repeated unnecessarily.
If you remove accidental duplication too soon, you might introduce a wrong abstraction that forces unrelated parts of your system to depend on the same logic. This makes the code harder to understand and modify.
“Code duplication may happen. Duplicating code per se is not bad. Duplicating behaviors is bad. You don't want two occurrences of the same behavior because when you change one (or fix it), that's where bugs happen.”
Alexandre Walsh
VP of Engineering at Axify
The best approach is to wait until a pattern emerges before duplicate code refactoring. If a single function keeps getting duplicated, you can extract it into separate methods or a separate class when it makes sense.
How to Find Code Duplication
Finding code duplication issues early saves you from unnecessary maintenance headaches. You don’t want to spend hours fixing the same bug in multiple places because identical logic is scattered across your code bases. The best way to detect duplication is by using code analysis tools that scan your project and highlight repeated blocks of code.
Here are some tools that can help:
- JetBrains IDEs (IntelliJ IDEA, PyCharm, WebStorm): These tools automatically detect repeated code patterns, so you can refactor or separate functions for better readability.
- SonarQube: A widely used tool that provides detailed duplication metrics, including repetitive lines and code blocks. It also makes it easier to track and reduce redundancy.
- Qodana (JetBrains' code quality tool): Helps you monitor duplication and other quality metrics across your software development projects.
These tools advise aiming for less than 5% duplication to ensure a maintainable code structure. Using these tools and best code review practices ensures your project stays clean and scalable.
Regular code reviews help uncover poor coding patterns, allowing teams to fix mistakes that impact clarity and maintainability. Modern development environments thrive on clean, efficient code, and reducing code duplication is a key strategy for lowering costs and improving scalability.
How Do You Fix Code Duplication? Duplicate Code Refactoring Explained
Fixing code duplication means refactoring strategically so your codebase stays clear and maintainable. If you try to force everything into shared logic, you risk creating code smells or making updates harder in the future.
The best way to handle this issue is to use duplicate code refactoring techniques, which improve code readability while keeping logic flexible. Here are the most effective ways to fix code duplication.
Extract Method for Duplicate Code Refactoring
If the same code appears in multiple places within a class, you should extract it using a separate method. This keeps your logic reusable and avoids knowledge duplication.
Example (Before Refactoring)
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python CopyEditdef calculate_discount(price): discount = price * 0.10 return price - discount def calculate_final_price(price): discount = price * 0.10 return price - discount + 5 # Additional processing |
Example (After Refactoring with Extract Method)
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python CopyEditdef apply_discount(price): return price - (price * 0.10) def calculate_final_price(price): return apply_discount(price) + 5 |
Now, any discount logic changes only need to be updated in one place instead of two.
Pull Up Method for Duplicate Code Refactoring
When two subclasses have methods that perform the same work, you should make them identical and move them up to a parent class instead of repeating them.
Example (Before Refactoring)
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java CopyEditclass Employee { double calculateBonus(double salary) { return 0; // Base class has no bonus logic } } class Manager extends Employee { double calculateBonus(double salary) { return salary * 0.15; // Bonus calculation duplicated in different roles } } class Developer extends Employee { double calculateBonus(double salary) { return salary * 0.10; } } |
Example (After Refactoring with Pull Up Method)
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java CopyEditclass Employee { double calculateBonus(double salary, double rate) { return salary * rate; } } class Manager extends Employee { double getBonus(double salary) { return calculateBonus(salary, 0.15); } } class Developer extends Employee { double getBonus(double salary) { return calculateBonus(salary, 0.10); } } |
This keeps the core logic in the base class while allowing subclasses to use different rates.
Form Template Method for Duplicate Code Refactoring
If two methods do the same thing but use different steps, use the form template method to extract common behavior.
Example (Before Refactoring)
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java CopyEditclass OrderProcessor { void processOnlineOrder() { verifyPayment(); sendEmailConfirmation(); shipOrder(); } void processInStoreOrder() { verifyPayment(); printReceipt(); handOverProduct(); } } |
Example (After Refactoring with Form Template Method)
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java CopyEditabstract class OrderProcessor { void processOrder() { verifyPayment(); sendReceipt(); finalizeOrder(); } abstract void sendReceipt(); abstract void finalizeOrder(); } class OnlineOrder extends OrderProcessor { void sendReceipt() { sendEmailConfirmation(); } void finalizeOrder() { shipOrder(); } } class InStoreOrder extends OrderProcessor { void sendReceipt() { printReceipt(); } void finalizeOrder() { handOverProduct(); } } |
Now, both order types follow the same structured flow without duplicating logic.
Extract Superclass to Support Duplicate Code Refactoring
If two classes share a lot of logic but aren’t part of a hierarchy, you should extract a common superclass.
Example (Before Refactoring)
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python CopyEditclass Dog: def speak(self): return "Bark" def eat(self): return "Eating dog food" class Cat: def speak(self): return "Meow" def eat(self): return "Eating cat food" |
Example (After Refactoring with Extract Superclass)
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python CopyEditclass Animal: def eat(self): return "Eating food" class Dog(Animal): def speak(self): return "Bark" class Cat(Animal): def speak(self): return "Meow" |
This removes redundancy while keeping the flexibility to define different behaviors.
Consolidate Duplicate Conditionals for Duplicate Code Refactoring
If the same conditional logic appears in multiple places, you should merge them into a single function.
Example (Before Refactoring)
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if (user.role === "admin" || user.role === "manager") { grantAccess(); } if (user.role === "admin" || user.role === "manager") { showSettings(); } |
Example (After Refactoring with Consolidated Conditionals)
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function isPrivilegedUser(user) { return user.role === "admin" || user.role === "manager"; } if (isPrivilegedUser(user)) { grantAccess(); showSettings(); } |
Streamline Engineering Efficiency with Axify
Code duplication isn’t just about clutter—it slows development, increases maintenance workload, and makes refactoring harder to track. Axify doesn’t eliminate duplication, but it helps teams measure engineering efficiency through DORA metrics, making it easier to assess the impact of refactoring and workflow changes.
"DORA metrics measure speed and stability. So, what is the impact of your duplicate code refactoring? Are you increasing stability over time with fewer bugs?"Alexandre Walsh
VP of Engineering at Axify
Companies we work with appreciate our DORA dashboard because it helps them monitor deployment frequency, change failure rate, and lead time for changes—all critical for improving development workflows and maintaining cleaner, more reliable code.
Beyond DORA, Axify provides additional tools that support a more efficient engineering process:
- Value Stream Mapping helps you identify bottlenecks slowing down development.
- Daily Digest keeps you on top of aging tasks so nothing gets left behind.
- Software Delivery Forecast helps you plan releases based on real-time data.
Duplicate code refactoring and workflow improvements only work when you have visibility into all your processes.
Besides, you need to do more than fix duplicate code—you need to make your team more efficient. See how Axify helps you move faster. Book a demo today to see how it works for your team.