I ran into a problem at my house i built in Falkreath. After i finished the dawn guard quest line i got the ancient falmer armor. I took it to my house and went down to the cellar. I took my Nightingale armor off my mannequin and put the ancient falmer armor on it. After i did a few quests i picked up some daedric armor and decided to replace the falmer armor. i took the falmer armor off and placed it into a nearby chest and left. Now every time i come back to my house i have 2 sets of ancient falmer armor on the mannequin along with the daedric armor no matter how much i remove either armor i have 2 sets of ancient falmer along with what ever is on the mannequin.

I know i probably shouldn't be complaining about this but i now have over 200,000 gold

But i still have 20 sets of ancient falmer armor and its bugging me.

Is there any way to stop it?

There isn't really any reliable solution to the mannequin armor duplication bug other than to never use them. It occurs due a scripting bug that Bethesda never bothered to fix and sadly never will fix.

That being said the below steps (on the page linked below) have worked for some people. There's no guarantee that they will work for you though.

http://elderscrolls.wikia.com/wiki/Forum:Skyrim:How_to_fix_armor_mannequin_glitch.