To reproduce:
Run the following commands:

$ mkdir test
$ cd test
$ touch thisTest.txt this_test.txt
$ ls
thisTest.txt this_test.txt
$ stdrename -s
$ ls
this_test.txt

Observed behavior:
If you have two different files which have the same name but different naming conventions, when you use stdrename to change them to have the same naming convention one will get overwritten.

Expected behavior:
Either the name doesn't get changed or something is added to the name to avoid the collision, e.g. "this_test.txt" and "this_test_1.txt" (just a quick example; there may be better naming scheme ideas).