The correct phrase is "Those who know, know."
In this phrase, the word "know" is repeated to emphasize that those who have knowledge or understanding of something will recognize or understand a particular situation or concept. The comma between the two instances of "know" indicates a pause, making the phrase grammatically correct.
Example: "Only those who know, know the true meaning of love."
The phrase essentially means that true understanding or knowledge of something comes from direct experience or deep insight, and those who possess that understanding will recognize it when they encounter it again.