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+# Code Reuse Thinking Guide
+
+> **Purpose**: Stop and think before creating new code - does it already exist?
+
+---
+
+## The Problem
+
+**Duplicated code is the #1 source of inconsistency bugs.**
+
+When you copy-paste or rewrite existing logic:
+- Bug fixes don't propagate
+- Behavior diverges over time
+- Codebase becomes harder to understand
+
+---
+
+## Before Writing New Code
+
+### Step 1: Search First
+
+```bash
+# Search for similar function names
+grep -r "functionName" .
+
+# Search for similar logic
+grep -r "keyword" .
+```
+
+### Step 2: Ask These Questions
+
+| Question | If Yes... |
+|----------|-----------|
+| Does a similar function exist? | Use or extend it |
+| Is this pattern used elsewhere? | Follow the existing pattern |
+| Could this be a shared utility? | Create it in the right place |
+| Am I copying code from another file? | **STOP** - extract to shared |
+
+---
+
+## Common Duplication Patterns
+
+### Pattern 1: Copy-Paste Functions
+
+**Bad**: Copying a validation function to another file
+
+**Good**: Extract to shared utilities, import where needed
+
+### Pattern 2: Similar Components
+
+**Bad**: Creating a new component that's 80% similar to existing
+
+**Good**: Extend existing component with props/variants
+
+### Pattern 3: Repeated Constants
+
+**Bad**: Defining the same constant in multiple files
+
+**Good**: Single source of truth, import everywhere
+
+---
+
+## When to Abstract
+
+**Abstract when**:
+- Same code appears 3+ times
+- Logic is complex enough to have bugs
+- Multiple people might need this
+
+**Don't abstract when**:
+- Only used once
+- Trivial one-liner
+- Abstraction would be more complex than duplication
+
+---
+
+## After Batch Modifications
+
+When you've made similar changes to multiple files:
+
+1. **Review**: Did you catch all instances?
+2. **Search**: Run grep to find any missed
+3. **Consider**: Should this be abstracted?
+
+---
+
+## Gotcha: Asymmetric Mechanisms Producing Same Output
+
+**Problem**: When two different mechanisms must produce the same file set (e.g., recursive directory copy for init vs. manual `files.set()` for update), structural changes (renaming, moving, adding subdirectories) only propagate through the automatic mechanism. The manual one silently drifts.
+
+**Symptom**: Init works perfectly, but update creates files at wrong paths or misses files entirely.
+
+**Prevention checklist**:
+- [ ] When migrating directory structures, search for ALL code paths that reference the old structure
+- [ ] If one path is auto-derived (glob/copy) and another is manually listed, the manual one needs updating
+- [ ] Add a regression test that compares outputs from both mechanisms
+
+---
+
+## Checklist Before Commit
+
+- [ ] Searched for existing similar code
+- [ ] No copy-pasted logic that should be shared
+- [ ] Constants defined in one place
+- [ ] Similar patterns follow same structure