Remove duplicated `Union` import

Currently, when `Optional` is found on the import statement, it's just replaced by `Union`.

Although `isort` (I think), removes the duplication statement, it would be cool to not generate a duplicated one for those who don't use `isort`...

Locate the logic that handles Optional in import statements and inspect how it replaces Optional with Union. Reproduce an import containing Optional and an existing Union, then verify the generated import contains Union only once; update the relevant test if the repository has coverage for this behavior.

Written by the indexing model from the issue text.