Rule for omitting duplicates in bruteforce attack?

[veggiebiker2500]

Yeah, I was just using a general example and didn't realize that a 12 char passcode would take years even on a gaming PC. But lets assume it's only a 6 char password...
hashcat64.exe -m 2500 -a 3 capture.hccapx -1 ?u?d ?1?1?1?1?1?1

What should I add to make it eliminate dupes?