Gods give boons to different people based on what they ask but what if

• Person A gets a boon that he will kill everyone he wars against in the next 10 years.
• Person B gets a boon that he cannot be killed in the next 10 years.

Now if Person A faces Person B in war, what will happen? Both have the power of boon but boon of both cannot be fulfilled. It's a deadlock, what is the solution to such situations?

Is there any mention in scriptures where there has been a clash between boons and how that situation got resolved?