If we hit the Duplicate button, we get a confirmation prompt with the name of the file that's been selected for duplication:

Cancel will close the dialog. Duplicate will create another copy of the file with an appended copy number, as shown in the following screenshot. The original filename has been used with the addition of -Copyn in the filename, where n is the copy number. Note the original file extension, .py, has been maintained in the new file: