My real estate agent inadvertently left out a contingent clause in the listing agreement and I received a full offer in two days. I wanted a clause in the contract that the sale of my property was contingent on closing on my replacement property. Do I legally have to accept the offer on my property?

If you have already signed the offer then you have accepted the current offer. However, you can now counter offer with your contingency and they can either accept it or pass. Again if you did not sign the offer the ball is now in your court it sounds like.

When did you discover the missing contingency - before or after you signed the listing? Have your agent amend the listing contract now. I would try countering the offer with the contingency first. In the future, if the contract is not exactly the way you want it to be DO NOT SIGN IT.