Duplicate Detection Rules

Core Principle

Duplication is the enemy of change. When you change one, you must change all copies. Miss one, and you have a bug.

---

1. Clone Code

1.1 Type-1: Exact Clone

Definition: Exactly identical code excluding whitespace/comments

Detection Criteria:

• 5+ consecutive identical lines
• 100% token sequence match

Severity: CRITICAL

Fix Strategy:

• Extract to common function
• Separate into utility module

1.2 Type-2: Parameterized Clone

Definition: Same structure with only different variable names/literals

// Clone A
const result = users.filter(u => u.age > 18);
return result.map(u => u.name);

// Clone B
const data = items.filter(i => i.price > 100);
return data.map(i => i.title);

Detection Criteria:

• Structural similarity > 90%
• Identical token type sequence

Severity: HIGH

Fix Strategy:

• Extract parameterized function
• Use generics/templates

1.3 Type-3: Similar Structure Clone

Definition: Similar code with some lines added/deleted/modified

Detection Criteria:

• Structural similarity > 70%
• Same core logic pattern

Severity: MEDIUM

Fix Strategy:

• Convert to strategy pattern
• Apply template method pattern

---

2. Role Duplication

2.1 Same Responsibility Classes

Detection Signals:

Signal	Description	Duplication Probability
Similar names	*Manager, *Handler, *Service	High
Same interface	Implements same interface	Very High
Similar method set	3+ same method names	High
Same dependencies	Depends on same classes	Medium

Severity: HIGH

Fix Strategy:

• Consolidate into one
• Redefine roles and separate

2.2 Synonym Usage

Synonym pairs to watch:

Group	Synonyms
Retrieve	get, fetch, retrieve, find, load
Create	create, make, build, generate, new
Validate	validate, check, verify, ensure
Process	process, handle, execute, run, do
Transform	convert, transform, parse, map, to

Rules:

• Use only one synonym per project
• Convention documentation required

---

3. Variable/Constant Duplication

3.1 Magic Numbers

Prohibited Pattern:

// Bad
if (retryCount > 3) { ... }
setTimeout(() => {}, 5000);
if (users.length > 100) { ... }

Fix:

// Good
const MAX_RETRIES = 3;
const TIMEOUT_MS = 5000;
const MAX_USERS = 100;

3.2 Duplicate Strings

Detection Criteria:

• Same string used 3+ times
• String length 5+ characters

Exceptions:

• Empty string ""
• Single character " ", "," etc.
• Logging messages (but error codes should be constants)

3.3 Duplicate Config Values

Detection Pattern:

// Duplicate config
const timeout1 = 30000;
const timeout2 = 30000;
const requestTimeout = 30000;

Fix:

// Single config
const CONFIG = {
  TIMEOUT_MS: 30000
};

---

4. Duplication Thresholds

Detection Thresholds

Type	Threshold	Reporting Condition
Type-1 Clone	5 lines	Always report
Type-2 Clone	90% similarity	Always report
Type-3 Clone	70% similarity	10+ lines only
Role duplication	3 same methods	Always report
Magic numbers	2+ usages	Always report
Duplicate strings	3+ usages	Always report

Severity Criteria

Severity	Definition	Action Deadline
CRITICAL	Immediate fix required	Current task
HIGH	Quick fix recommended	This sprint
MEDIUM	Planning required	Next sprint
LOW	Awareness only	When time permits

---

5. Exceptions

Allowed Duplication

Situation	Reason
Test code	Maintain test independence
Generated code	Auto-generation tool responsibility
External interface implementation	Contract compliance required
Performance optimization code	Intentional inlining

Not Considered Duplication

• Code 2 lines or less
• Idiomatic patterns (e.g., if (err) return;)
• Language boilerplate

---

6. Fix Patterns

Clone Code Consolidation

1. Identify common parts
2. Parameterize differences
3. Extract common function/method
4. Replace originals with calls
5. Run tests

Role Duplication Consolidation

1. Analyze responsibilities of both classes
2. Determine consolidation target (by reference count)
3. Merge methods
4. Migrate references
5. Remove deletion target

Constantization

1. Identify duplicate values
2. Assign meaningful names
3. Add constant definitions
4. Replace usages
5. Code review

---

7. Metrics

Code Duplication Rate

Duplication Rate = (Duplicate Lines / Total Lines) × 100%

Target: < 5%
Warning: 5% ~ 10%
Danger: > 10%

DRY Score

DRY Score = 100 - Duplication Rate

Excellent: > 95
Good: 90 ~ 95
Needs Improvement: < 90