Scan for Added Roms does not check for alternate revisions.

[The T]

When choosing Scan for Added Roms, it imports any games as a new game, even if versions of that game already appear in the list. (So, say, if I found a Europe version of a ROM, it adds it separately from the additional US version I already have.)

[Retro808]

This is not a bug but would need to be a feature request as the scan option is not designed to combine with existing. Neither does the drag and drop feature. Combine only works when you are importing roms at the same time. LB will look at the one being imported and combine them at that point.

You can edit the ticket and make a feature request so users can the vote on it as well.