when you've seen, heard, etc. one, you've seen, heard, etc. them all
when you've seen one (something), you've seen them all
All of these things are exactly alike; experiencing one of these things is enough to know what everything else of its kind will be like. Verbs other than "seen" are often used, such as "heard," "read," etc. I've never understood the appeal of all these different boy bands—once you've heard one, you've heard them all. I prefer dramas, if I'm honest. Once you've seen one action movie, you've seen them all, as far as I'm concerned.
when you’ve seen, heard, etc. ˈone, you’ve seen, heard, etc. them ˈall(saying) used to say that all types of the things mentioned are very similar: I don’t like science fiction novels much. When you’ve read one, you’ve read them all.
See also: all