On HiNative.com a Taiwanese person asked about the meaning of "cool", as in "cool friend" or "cool girl". I answered:
It means you admire something about them and would like to know them (if you don't already).
A Brazilian person asked if I should have used "yet" instead of "already". I replied that in this instance I believed either was okay, but "yet" would have been slightly better. He asked for further clarification, because he had learnt that negative sentences or questions had to use "yet".
I'll quote my answer below and would appreciate feedback about both the validity of my answer as well as my reasoning.
You have learnt correctly, and the page I linked to says it as well. Languages sometimes are fuzzy, and a good general rule can sometimes be bent. However, it was not my intention to "bend the rules", and in truth I did not even think about it as I wrote it. Since you are challenging me to think about this (in a good way, thank you) here is what may have gone thru my mind.
1) I was typing on my smartphone at the time, and I chose to shorten the parenthetical phrase. So, instead of typing "(if you don't already know them)", I typed "(if you don't already)". To my ear, it sounds better to end the hanging phrase with "already" than "yet". Other ears may differ. :)
2) Look at the entire thought.
- It means you admire something about them and would like to know them (if you don't already)."
Even though "not" is there, the thought is positive -- you would like to know them. The conditional parenthetical thought does not change the tone of that idea.
That all said, yes, "yet" would have been better. It would also have been better to complete the hanging parenthetical phrase. But it is what it is, and I do not believe what I wrote was actually incorrect.
If I am wrong, let me know so that I may correct myself in that thread.