## 17 characters

> **One bar bars one**.

- The first *one* is a single instance of something
- The first *bar* is a place where you are served drinks
- The second *bar* means to prohibit someone from being served
- The second *one* is the general third person pronoun, referring to all people in general

You could imagine a context something like this:

> One bar bars one, and suddenly one gets a reputation for being a troublemaker!

If you don't like the fact that the second *bar* has an s on the end (several other answers have "modified" forms like this, but I'm a bit of a perfectionist...), you could have

> One bar did bar one.

which has an extra 4 characters, bringing the total to 21.