Which is the correct usage. 'Who is fooling who?' or 'Who is fooling whom?'
Who's fooling who? -- casual conversational English

Who is fooling whom? -- formal, careful English
Technically, the second pronoun should be whom, but it certainly doesn't give the effect of the more common who. I would liken this to a sort of poetic license.
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Who is fooling who
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You still need to use a question mark!