What's the difference between ないことはない and ないこともない？


I found these two grammatical forms, ないことはない and ないこともない. To me they seem
really similar both in usage and meaning, am I missing something? If somebody
could give me some examples too, it would be really helpful.

Edit

I might be wrong but in the そうまとめN2, ないことはない and ないこともない are listed as two
different forms and with two different meanings, as in: 1)やめられないこともない。 it is
possible to quit 2)覚えられないこともない。 it is not impossible to remember

