I have completed the game on normal difficulty and I want to reply the game on hard. If I play again in new game + do I get to keep all of the weapons that I have collected in the previous playthrough?
Sort of. When starting new game + you will keep upgrades and such, but you will obtain weapons in the same order as before. So if you fully upgrade a particular weapon in the first play, you won’t have it on you at the start of the next game, but when you finally obtain that weapon in the next playthrough, all of the upgrades you purchased will still be unlocked.