You didn't provide much data, but if you split payments on the first 2 MO's (500 and 500, and 500 and 500), and then tried to do it a third time, that seem suspicious to the clerk. Best case is just an ID check to scare you off (which seems to have worked). Worse case they log your info. I've had it done to me on a second split 500, CSR just looked at ID, I inquired as to why, his response was "transaction seemed odd", I responded with "oh, ok, I've done this previously without issue - if it's a problem I certainly won't do it again".

Morale, don't get greedy, and learn your clerks and what THEY are comfortable with, in addition to what YOU are comfortable with.