I'm sure you've had this conundrum: You're chatting with a friend or colleague, and they start repeating a joke or story they've told you before. Yikes, now you have a tough choice: 1) Do you interrupt and say they've told you before? Nah, that's rude. (2) Do you let them continue with their joke, and then fake a laugh? Nah, that's insincere. (Besides, halfway through, they might remember that they told it to you before. Now they feel foolish and think you're pandering them.) So what's a civilized person to do?
Here's salvation. Simply let them finish the story and then say, "That's a GREAT story!" (This is appropriate whether you heard it or not.) Your reaction pleases them, and you're freed from being rude or faking a reaction.
"But, wait," you might ask, "what if, after their story, they remember you’ve heard it?” They ask why you let them continue. No problem. Simply say, "I enjoyed the story so much the first time that I wanted to hear it again!"