Questions with multi-syllable verbs, e.g. 你认识不认识他？ vs. 你认不认识他？ What would be the difference between 你认识不认识他？ and 你认不认识他？ Are they the translated with the same "Do you probably know him?
Basically they have the same meaning, but the former is a little more formal. And in spoken language the latter is often used because the former twists the tongue a little bit.