The optimum approach would be not to attempt to create duplicates in the first place.
In fact, you do not create duplicates as there is some kind of automatism.
If you insist on your solution, you can use this approach:
First rename all files and add a (2) for every file in the Converter mask. The duplicates will stay as they are and will not be renamed.
Then rename the files again to the originally intended format (without the (2)) and now all files will be renamed except those which would create a duplicate.
Now you have only those with an added (2) that would lead to a duplicate.
The problem with that approach is still that you rob yourself of a valuable indicator that there is a flaw in the projected name and that perhaps the tags need a little refinement - so that no duplicates get created (which they aren't anyway, but we had that already).