There's a bunch of "words" in the english legal system that are like that: a pair of words (with one frequently sourced from french). There's a list here: https://en.wikipedia.org/wiki/Legal_doublet

"immaterial and impertinent" isn't referenced on that list, but shows up in a list of reasons to dismiss. https://www.upcounsel.com/legal-def-averment suggests that they were once different, but are now the same.