Wouldn't that indicate an issue in your business logic attempting to do this in the first place?

Or if you are relying on DB to fail and your business side to detect and react, you'd still have that built into the business logic so you can just keep retrying the schema migration until it succeeds (if it's rare this happens).

So while I can see how this can happen, it basically is a bug and it means you are doing the migration yet the invariants are not going to be satisfied. Basically, even if it succeeds, you will have future inserts fail with unique constraint being broken.