I think it will be to complicated to lookup for allready made Mxx number at same file in the adding process. It will prolly also slow down the media adding process.
No, not complicated and can not slow anything down, because it is already part of the code, and has been for ever. This is why I do not understand (yet) how it can happen.
Hopefully tomorrow I will know better. If it can happen it is a bug which must be fixed. The system is designed for multiple users editing at the same time. This is more important (to me) than any check after the damage is done.