Whenever I have done that and don’t want to overwrite, I do either of the following -
1/ Select NO to exit with no backup and then run RootsMagic again, and exit changing the file name!
2/ When on the Confirm Overwrite screen, open Windows Explorer, Navigate to where my backup file is, then chenge its name (By saying adding a _1 to the filename, then back in RM Click YES to overwrite, it will create a new file.