This error can happen if different regions in your session are referencing different copies of the same file on multiple drives. when you go to copy the session, it tries to put two copies of a single file in your new audio files folder.To eliminate such a conflict, have Pro Tools reference a single audio files folder by manually moving copies of all the audio files (being referenced on multiple drives). To do this:-Rename all the folders that are currently referenced and then when opening your session.-Every time Pro Tools asks for a file that isn't in your audio files folder, copy it in. Although somewhat time consuming, this is the most effective means of bypassing the error with the current version of Pro Tools (PT TDM 5.3.1).