What the two 'ones' stand for in 'It takes one to know one'?

[Haihao]

Hi,

Could you tell me what the two 'ones' stand for in 'It takes one to know one'?

Thank you.

Haihao

[Yankee]

Hi Haihao

That means "one of that type of person". This phrase is usually derogative.
A person with certain (negative) personality traits recognizes another person with the same traits:
Sam: You're an idiot.
John: It takes one to know one.

Amy

[Pamela]

Another analogue of the proverb:

Birds of a feather flock together.

[Alan]

Oh and another thought: It takes a thief to catch a thief.

A
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[Tom]