Double Negation :

All parents are forever correcting their children for the find of double negatives,  there we have to be very alert with them in natural language like: "He didn't tell me not to do it" doesn't essentially means the same as "He did tell me to do it". So there is same true with logical sentences: that we cannot, let see example, change ¬(P ^ Q) to (¬ P ^ ¬Q) without risking the meaning of the sentence changing. Moreover, when we can alter expressions with negation there are certain cases. But there is two possibilities are given by de Morgan's law below, so we can also justify statements by removing double negation. Thus there are some cases when a proposition has two negation signs in front of it, like : ¬¬P. 

Here you may be questioning that why on earth anyone would ever write down a sentence with just like a double negation in the first place. But really, you're right. As there humans, so we wouldn't write a sentence in logic such that. Wherever, remember that our agent will be doing search utilising rewrite rules. So there it may be that as part of the search, but they introduce a double negation, through following a particular rewrite rule to the letter.