Code duplication detection as a vibe-check structural gate

[jflowers]

Summary

Add code duplication detection as a structural quality gate in vibe-check, integrated into the vibe-check analyze command with a configurable duplication threshold.

RFC: https://github.com/orgs/unbound-force/discussions/483 — Group 8
Phase: P2 (Accessibility)

Context

Duplicated code is a significant maintainability risk, especially with agent-written code where agents may not be aware of existing implementations. Detection gives agents and reviewers visibility into redundancy.

Implementation

vibe-check Integration

• Add duplication detection to vibe-check analyze
• Use AST-based structural similarity for Go (leveraging go/ast)
• Configurable threshold: max percentage of duplicated code
• Report duplicated blocks with file locations and similarity scores

CI Gate

vibe-check analyze ./... --max-duplication=5

Exit code non-zero if duplication exceeds the threshold.

Output

• Duplication percentage per package
• List of duplicated code blocks with locations
• Included in the metrics.ModuleGraph output (via warnings or a dedicated field)

Acceptance Criteria

• Duplication detection integrated into vibe-check analyze
• AST-based structural similarity for Go
• --max-duplication flag with configurable threshold (default: 5%)
• CI fails when duplication exceeds threshold
• Duplication report included in JSON output
• Supports AD-008 (DRY) from the convention pack
• Tests with sample codebases containing known duplicates

Dependencies

• Depends on: vibe-check analyze — Go-native package-level coupling metrics #2 — vibe-check analyze command
• Supports: Agent-design convention pack AD-001 through AD-010 (Group 3) #4 — Agent-design convention pack AD-008 (DRY)