I got my two royal pistols the double barrel pistols and every time I fast travel anywhere it always changes one of my pistols to the Scottish flintlock pistols but it keeps my other royal pistol how can I keep both of them
This is a known bug. It happened on consoles and even on pc.
It's just a bug that Ubisoft has to patch. But since it's not game breaking it isn't that much of a big deal.
I have fixed it. It's easy,but it only works on the royal pistol. This worked for all my friends; you just have to go to the manor and select the royal pistol(when you are changing your pistol you should be in the assassin white outfit). Immediately after changing go and change your outfit to achilles first assassin outfit. When you equipped the outfit just immediately shutdown your console/pc and restart. This worked for all my friends who were using pc and ps3. Enjoy the game.