So I found out a duplication bug that I encountered while building custom houses. The glitch is active only when the custom building is incomplete. If you try to move objects that have already been placed in the building the item will be duplicated. This also Occurred when using the preset tier two Smithing house, which my Hearthlings where unable to finish due to the design flaw with the last diamond window block. Has anyoen ever come to this issue before?
Welcome to the forum, @Daizukun 
We don’t support modifying incomplete buildings. If you move objects there, they will become a new part of the building template. I know it’s not explained anywhere in the game, that’s our fault.
But we’re currently reworking the entire building editor so these bugs will get fixed at some point.