Add a warning and an option to create a new file name when two identical export file names are in the rendering queue

Currently, if two file names for export in the queue are identical (in the same directory), the second rendering round will overwrite the first. IMHO, there should be a warning either before adding the second identical file and directory name to the queue, or the automatic opening of a new dialog to save the second file under a new name (just like it currently works with already existent files in the output directory.

Use case: I recently cut only the actual music parts from a musical video (not knowing the titles of the songs yet), and inadvertently made a numbering error, so I had two files with something like “song_03” in my queue. Once I found the real titles, it caused a lot of confusion until I found that the first file was overwritten without a warning.