Both payments can exist on one house, and whether they should is a question your written agreement has to answer before the house exists.
Start with what each one is. The assignment spread is what you earn as a principal: you hold a purchase contract at 150 and sell your rights in it for 165, so the $15,000 is the price of your contract. The acquisition fee is payment for a service you performed for the client. They come from different places, so nothing about them is automatically mutually exclusive.
"Double dipping" is what people call it when the buyer didn't know about the second payment. That's the real problem. A buyer who agreed to a $5,000 fee for sourcing, and who then discovers you also made $15,000 on the contract they thought you were assigning at cost, will not stay a client, and depending on the state and on what your agreement says about your duties to them, they may have a legal claim. Several states have also tightened disclosure rules on assignments specifically, so what has to be told to whom varies by state and a local real estate attorney should read your paperwork.
The clean version most sourcing agreements use: define one compensation method per property, and say plainly whether the spread is credited against the fee. So a house you assign pays the spread only, and a house the client contracts directly pays the fee. Ambiguity here always resolves in the client's favor once they're annoyed.
