Yeah, Preston really needs to be tweaked so that he only gives a mission if asked for one and only gives one at a time. Once you have all the settlements he should just stop giving you quests.
Same goes with the Castle. Even if I already have Preston quests, the castle announcement will just continually pile them on.
My game weirdly glitched out so that every time I spoke to him the conversation would start with my character stating that she was sorry that she couldn't defend against some supermutant attack, then he would give me another quest line but no quest would pop up. This was fine with me except that this conversation would take place no matter the situation. Even in the middle of quests with other dialogue... Like standing on the roof about to blow up the Institute.. "Sorry preston, I couldn't defend against the supermutants." "oh well, here's another quest.".. "Now all you have to do is press that button."