Opus has the nickname "Slopus" in a lot of circles for a reason. It can write nice code in isolation, but the way it organizes that code and its rigor in addressing edge cases/making sure things are robust leave a lot to be desired. Opus is particularly famous for having a real problem reinventing stuff that already existed in the codebase because it wanted to get to work before exploring sufficiently.

what you're describing doesn't sound like such a big deal -- it's (A) obvious during review, (B) easy to fix in a single prompt, (C) simple enough to fix manually, (D) can be mitigated with tokenmaxxing (agent review passes, prompting, subagents, etc)

regarding edge cases -- less is more in my experience, as removing is harder than adding