[completions] Exclude duplicate class base completions

### Summary

I would not expect `Foo` to be suggested here when Foo is already set.

<img width="662" height="178" alt="Image" src="https://github.com/user-attachments/assets/1c20a6ed-9589-4c60-94c2-04c39e019198" />

### Version

latest main (78d134358)