This question already has an answer here:
Is there a pair of words in English, where one is a grammatical negation of the other, but they are synonyms?
For the negation I'm thinking about a negative prefix or suffix, like de-, dis- or -less, but it can also be built in other way.
For example for the word 'likely' the hypothetical pair would be 'likely' and 'unlikely' and they would mean exactly the same (i.e. 'likely').
This came to my mind because there are at least two such pairs in my native language.
Edit: IMO this is not a duplicate of auto-antonym. The auto-antonym examples from the linked pages don't meet my criteria. However I was unaware that list questions are automatically "too broad". Thanks Andrew Leach for explaining me that, I think I will try a community wiki question then.