Create User Avoid Duplicates

Create User Avoid Duplicates

Looking to create users and avoid duplicates. Created new format (first initial of first name and first five letters of last name) Example- Stephen Rosalbo = srosal
Need to check for duplicate and if so add 01 or 02 etc. I edited Login (pre-2000 Windows) to check for duplicate. other settings I am not sure what to choose and seems whatever I do chose still comes back with error that account exists.

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