〜ないことはない

A double negative, ないことはない means "it's not that … not": 行けないことはないが、ちょっと遠い, "I could go, but it's a bit far".

It's a reluctant yes. The speaker admits something is possible or true, but with reservations, and often a けど or が follows with the catch.

It's very common with potential verbs: 食べられないことはない, "it's not that I can't eat it"; 間に合わないことはない, "we might just make it".

ないこともない is a softer version. Don't confuse it with ことはない alone (after a dictionary form), which means "there's no need to".