I bought a trailer house and the owners gave me the title to one of their other trailers and won't give me the title to my trailer?
Can I get a duplicate title or how do I go about getting the title in my name?

"Won't give" is a loaded phrase. You MUST have the right title or you never will be able to sell, and could even have your home sold right out from under you. You should not have handed them a dime without inspecting the title.
Do you have any clue why this happened?
Tell them you are suing, and then do so, I'm afraid, even though it's going to cost you money.

You could first contact the Consumer Protection Division of your state, if you are in the US.
You could second sue in Small Claims, although there's a low dollar limit, often about $10,000.
Any lawsuit over the limit will cost a lot.

Do you have a bill of sale? Why is it the wrong title? Why does he refuse to give you the correct one?