Does また来なくちゃ mean "I have to come again" or "I must not not come again"?

Question

So, does it suggest that I did not come last time or does it suggest that I did come last time?

Answer 1

There is no double negative here.

来なくちゃ is a standard contraction of 来なくては

The complete sentence would be

また来なくてはならない

So, the meaning is "I'll have to come again".

There is no suggestion that you came last time or not. You probably did, but it could be that a meeting was set up and canceled and now it's been set up again.

It's very much like saying "I'll have to come again". You can try to milk more information from this if you'd like, but there's no real reason, unless you know more about the situation, to presume whether you came previously on not.

A similar contraction is 来なきゃ for 来なければ.

In either case, the ならない, いけない, etc portion is often omitted.

Answer 2

また来なくちゃ implies you are in the target location now, you are about to leave and that you feel you have to go there again.

E.g.:

ハワイは本当に綺麗だね。また来なくちゃ (at the Hawaii beach).

え、電池忘れたの？また来なくちゃいけないじゃん (at a maintenance site)

You can't say it once you have departed (though it's ok if you are still early on your way back home, e.g. in the plane in Hawaii waters).