Seller accepted our offer… And someone else’s

We put in an offer on a home. They countered. We signed the counter offer and have gone through with home inspections, earnest money, etc.

We just found out that the seller had accepted another offer simultaneously – both are mutually agreed upon. Our agent, who we trust and has been nothing but extremely competent, says this is an incredibly rare situation.

Apparently the other buyers have lawyered up already and we’re considering doing the same.

What are our options? How common is this? How is this commonly resolved?

Edit: this is in Washington State, king county.