PS: Remember that you should only see the warning once if you change a whole block of file names at the same time.
For example, lets say that you've got
testing.mp4.001
testing.mp4.002
testing.mp4.003
etc, all the way through to
testing.mp4.100
If you select the whole block and then try to rename them to testing.mp4 you'll see the warning message once but you should then end up with
testing.mp4
testing(1).mp4
testing(2).mp4
etc, all the way through to
testing(99).mp4