That's a good question ... to which I can't give you the answer
In general, it's advised to remove files, so that it is easier later to differ files that belong to the database, and files that don't. Or, for other reasons I can't think of right now, but mostly creating unnecessary files (if any) in the same folder, maybe files that also end up in backups (file level backups) that should not, etc. etc.
Same as for a file restore (not being a database), it's always advised to clean the structure, before adding again.
I have done recovery of our PROD db to another server for running reports, just like in dataguard.
Then it's not a standby database.
I want refresh the database again for new backup copy.
Does RMAN recovery overwrites existing dbfs?
If "RMAN recovery" means "RMAN restore", then the answer is Yes. If not, please clarify.
Or do I need to remove them 1 by 1?
As above, the answer is No.