seen one, seen them all

used to indicate that all types of a certain thing are almost the same

The origin of the idiom "seen one, seen them all" is not attributed to a specific source but is a commonly used phrase expressing the idea that once you have witnessed or experienced one thing of a particular type, there is little variation or novelty in subsequent encounters.

I've been to a few music festivals, and honestly, seen one, seen them all—they tend to have similar stages, crowds, and performances.
After watching a few episodes of the TV show, I realized that the plotlines were repetitive, proving the point that seen one, seen them all.
Having visited multiple art galleries, I've come to the conclusion that when it comes to modern art, seen one, seen them all—they often feature similar abstract pieces.
My friend invited me to another corporate conference, but I declined, saying that I had attended enough in the past and seen one, seen them all.
I used to collect stamps, but after a while, the designs and themes became so repetitive that I lost interest—seen one, seen them all.