CS50X Problem Set 2 Substitution check50 not recognising edge case

[j0lev]

Check50 in CS50X PSet 2 Substitution does not check whether there are case insensitive duplicates. E.g. it does not check whether there is an 'a' and an 'A' in the key of the command line argument.

Therefore, it fails to check this requirement:

If the key is invalid ("[...] or not containing each letter exactly once), your program should print an error message of your choice (with printf) and return from main a value of 1 immediately.

Therefore, there should be a check in check50 prompting a command line argument with duplicate letters with one letter being lower case and one letter being uppercase. E.g. AaBCDEFGHIJKLMNOPQRSTUVWXY
with the expected result of exiting with 1 and outputting an error.

(please keep in mind this is my first time issuing a bug, so the format may not be accurate)