While renaming an error occurred: File already exists and overwrite not specified:

I am using a software called Photo Mechanic on a 27" iMac. It works very well on Mac apart from the above annoying error. I have checked the status of individual files it will not rename and they are unlocked and all permissions are set allow me to edit the files.

I suspect this is something to do with permissions on a iMac. Anyone come across a similar problem?