Stop me if you've heard this one.
The bartender is nonchalantly responding to a bar joke occurring in real life. "A rabbi, a priest, and a minister walk into a bar..." is a popular, stereotypical beginning of a bar joke. This sort of joke is commonly viewed as overdone, and the humorous part is that, even if it were happening in real life, a very jaded person might say something indicating their boredom at the overdone-ness of the situation, i.e., "stop me if you've heard this one" means "stop me if you have heard this stereotypical joke before."