请翻译这句not that that justifies treating them badly 为什么有两个that，以及justifies treating是什么意思？

A general habit of optimism in one person could make them intolerant of a colleague who's constantly pessimistic- not that that justifies treating them badly, of course.

原句如上，请问最后为什么有两个that，justifies treating 两个动词放在一起没读太懂。