At the moment, the same entity can be added several times to a series (as an exact duplicate). This is probably going to be blocked with MBS-8668 (while still allowing adding the same entity multiple times with different numbers) since it seems to be an error in most cases. That said, existing cases still will need to be cleaned up.

Additionally, merges can leave duplicates in series as well (MBS-9494). This is a case that often needs user review (especially for manual series).

A report to allow users to find and clean up series with duplicate entries seems like it would be quite useful given these issues.