All explanations must include the legal theory!

After a car accident between Kenny and Johnna, Kenny claims to have whiplash and says "If you don't pay me $5,000 right now, I will sue you for $10,000 for my damages!" Johnna, not wanting to go to court, gives Kenny the check to avoid the lawsuit. If Johnna wants to stop payment on the check and claim duress does she have a case? Please explain why or why not.