Comment 2 for bug 454591

Actually, upon reflection, always setting --allow-source-mismatch could be bad. A user might reasonably try to backup two computers to the same folder. With --allow-source-mismatch, the backups would keep flopping back and forth between a backup of one computer vs. the other. And the 11.0 solution (which asks the user what to do) is too invasive to backport. So I don't think I'll release a new 10.x version.

For the moment, I recommend a workaround of backing up to a different folder. You can always restore from the previous folder if you need to by pointing Deja Dup at it again.