Is there any way to restrict a user from entering a duplicate starting character?

Hi Amit

You could try salesforce validation rules, as you are saying to check duplicates charcters upto 3,4,5.. starting characters which means you can make use of following functions of validation rules

1. LEFT, RIGHT - For extracting starting characters
2. BEGINS- For checking starting charcters of string
3. CONTAINS- For checking any existence of specified string in that.

e.g Now string1: aabcd and string2: aaaxyz, so it shows that you only have to check for "aa" characters in between and it checks automatically for any number of same existence

Hope it helps

Thanks