Two different mechanics here, so the terms first. An assignment means you transfer your rights under the purchase contract to another buyer for a fee, the assignment fee, and you never take title. A double close means two separate closings, usually the same day. You buy from the seller on the first one and sell to your end buyer on the second, so title passes through you.
With a no-assignment clause you have two roads. Ask the seller for written consent to assign, which people skip because they assume it's a no and sometimes it isn't. Or double close, which the clause doesn't restrict because you're actually performing on the contract.
Yes, you pay closing costs twice. Your fee doesn't shrink in the sense that the $17k spread is still the spread, but the second set of costs comes out of it, often a couple thousand plus whatever the short-term funding costs, commonly a point or two of the purchase price with a minimum fee. So the number you keep is smaller than an assignment would have paid, and then you're splitting it with the member who brought the buyer.
Your earnest money isn't at risk from choosing between these two. It's at risk from the closing date. If the double close needs three weeks to arrange funding and your contract closes in twelve days, the deposit is what you lose. Get an extension in writing now, before the funding conversation.
One more piece worth raising with a local closing attorney rather than the group: several states have rules about disclosing that you intend to assign or resell, and what a non-licensee can be paid for, and those differ state to state.
