It's been said that items respawn after 10 days. If I loot everything, then it will essentially duplicate after 10 days?

All the items that came with the house, yes they will duplicate. What you put there yourself does not.

Not duplicate. An item from the same Leveled List will spawn at the location of the item you looted.


EDIT: Didn't see that this was Hearthfire...

Player house items from chests do not respawn.
But of course, if you keep visiting that house more than every 10 days, nothing will respawn at all.

If you're talking about ingredients and the like, yes, they are supposed to respawn after 10 days. Mine do at the 2 houses I don't have my family at and don't visit as regularly.

I've removed items from shelves and after a few days they come right back.

I hope that doesn't knock my stuff off...

It will if you removed something to use that spot, and are away from the house for 10 days. If you want to decorate yourself it is best to decorate around what is there. PC players can mod the game to get rid of the problem.