I've seen this in an English Learning Facebook page that we can say:
I can't not go I can't not tell her I couldn't not eat I couldn't not look
Are they true? Is it formal or informal? What structure is this?
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The structure is fine but it's not quite as simple as just a double negative.
"I couldn't not look" doesn't mean "I could look", it means "I had to look". Likewise for the others.
Put into some context, "I can't not tell her" might be said in the following conversation.
A: Did you hear Mary's boyfriend was at a strip club?
B: No, does she know already?
A: No she doesn't, so don't tell her.
B: I can't not tell her, I'm her best friend.
Again, "I can't not tell her" means "I must tell her", not just "I can tell her if I want to".