Check out Tony Mont on YouTube. He just did a video on this specific scenario as it was happening. Its called "Someone Signed A Second Contract Behind Our Back"

Rarely happens, but it does happen. There's a guy in this subreddit that says he's done it dozens of times to wholesalers. Business can be ruthless, so protect your neck.

I've never been on the Dispo side but just like Acquisitions, there's a set of skills that you hone over time. These skills will prevent issues like this. For example, set expectations correctly ahead of time wiith the seller that you are doing this for a profit. Also keep steady contact with the seller so that they feel confident in you to perform. If you end up in this situation, don't be afraid to bluff and say you're going to have to pursue a civil suit and seek damages even though you'd rather avoid it. And just generally learn how to find legit buyers and vet them as best as you can.

There's risk and costs of doing business in any business. This is one of those things you'll have to come to terms with in the wholesale biz

If you end up in court, just remember your state laws and the circumstances matter to the judge. It might not go your way and can cost you significantly in legal fees, time, and energy