Tente adicionar -d à sua linha de comando:
Collisions and Deletions
When any two or more matching files would have to be moved, copied, or linked to the same target filename, mmv detects the condition as an error before performing any actions. Furthermore, mmv checks if any of its actions will result in the destruction of existing files.
If the -d (delete) option is specified, all file deletions or overwrites are done silently.