I think leaving it as is, issuing a warning, or asking a user to confirm the action instead of failing is better, because I sometimes first do

<verbatim>
mv file renamedfile
</verbatim>

and then remember to mv it in Fossil:

<verbatim>
fossil mv file renamedfile
</verbatim>

If Fossil didn't allow this, I'd have to rename the file back to the original name, then do `fossil mv`, and rename it again.