Why does Julia sometimes compile the same function multiple times for the same arguments?

Because Julia specializes methods based on type signatures, world-age state, and invalidation caused by code changes.

Julia may recompile the same function when: (1) a new method is added that changes dispatch, (2) inferred types change due to new information, or (3) world-age invalidation occurs due to runtime method redefinition. Each unique stable environment of types and method tables can trigger a fresh compilation. This is essential for correctness but can cause hidden compilation overhead.