Signing foo.pdf ends up in foo_signed.pdf (with "signed" being translated into local language.) Adding a second signature produces foo_signed_signed.pdf. A third signature ends up with foo_signed_signed_signed.pdf and so on.
This is not really a good solution. We need to come with a better solution.
I'm going to assign this to @aheinecke - but @ebo might also have opinion on how it would be nice.
Once we have a decision, implementing it should be kind of easy.
How about adding "-2" to a document where before _signed already was in the name, i.e. foo_signed.pdf -> foo_signed-2.pdf and so on: foo_signed-3.pdf, ...
I know Microsoft is probably not the best example, but copying an already copied file with the Windows Explorer you get "Copy of Copy of Copy of originalname.txt". Moreover, I think foo_signed_signed_signed.pdf makes it pretty clear that this is a PDF that has been signed multiple times. I would leave it as-is. People who don't like the name can easily change it.
From my practical expexperience, @ebo's suggestion will work best for me. The other thing I have seen is to not use -signed but to append the initials of the signers.