fergius-engineering/instincts/no-duplicate-logic

The rule

Before you add logic to a function that already exists, read the whole function. The case you're about to write is often already handled a few lines down. A second block that produces the same result doesn't replace the first — both run, and the output silently doubles. The fix you needed was usually three lines added to the block that's already there, not a new block beside it.

This is not the same as fixing in the shared layer. That's about where a bug lives across callers. This is narrower: don't paste a parallel branch inside one function without reading what's already in it.

Fires when

Adding a case, branch, check, or emit to an existing function, handler, switch, or validation pass — especially when you jumped straight to the insertion point without reading the rest of the function.

How to apply

Before adding logic that emits a result keyed by some id or name, search the whole function for that id or that result. If a block already handles it, extend that block instead of adding a new one. Read the function top to bottom before you insert anything into it.

Worked example

A validation function already emits a "bad-name" issue for a record. You add a new block to emit "bad-name" without noticing the existing one a screen above. Now every bad-named record produces two identical issues, and a count somewhere is quietly off by a factor of two. The real change was three lines inside the block that was already there. Reading the function before inserting would have shown it.

Red flags