When you’ve seen, heard, etc. one, you’ve seen, heard, etc. them all
Definition 1 of 1
Pro Tip 1/3Cliche Warning
Using this phrase can suggest you think something is unoriginal or overdone. 
Another superhero movie? When you've seen one, you've seen them all.
Pro Tip 2/3Slight Disparagement
It often implies a mild criticism or lack of enthusiasm about the things being described. 
This town's pubs are all alike. When you’ve visited one, you’ve visited them all.
Pro Tip 3/3Not for Uniqueness
Avoid using this phrase when referring to things appreciated for their uniqueness or individuality. 
I like handcrafted items, so I don't agree that when you've seen one, you've seen them all.