I am aware of the concept of "personality clash", when two people can't get along because their natures are too different, but what is it called when two people can't get along because their personalities are too similar?
Learn English – Expression for two people whose similar personalities makes it difficult for them to get along
expressionsidiom-requestsidiomsnounspersonality
Best Answer
As Dan Bron says in a comment under the poster's question, one option is to use the expression "likes repel." Another option is to say that the their personalities leave the two people struggling with "contempt bred of familiarity."