Explaining "When you have seen one, you have seen them all"
What does it mean?

The phrase means that once you have seen one example of something, you feel as though you've seen all examples of it, suggesting a lack of variety or notable difference among them.
Tone
Dismissive
Origin
The exact origin of the phrase is unclear. It is a colloquial expression that likely developed over time to convey a sense of monotony when experiencing something repeatedly similar. The earliest recorded usage dates back to around 1811.
Examples of usage
- After visiting multiple chain hotels, John said, "When you have seen one, you have seen them all, because they all looked exactly the same."
- Sarah wasn't impressed with the series of paintings, remarking, "When you have seen one, you have seen them all; they're all just variations of the same theme."