You mean as in "You signify all that that/which I hate"? This is grammatically possible but unnecessary; I think the sentence is better without it. "represent" would be a more usual choice of verb.
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Debpriya DeShould there be a relative clause "that" or "which" after "that" in the sentence ?No. that I hate is already a relative clause. There's no point in taking that as a demonstative pronoun. all already gives you an antecedent for the relative pronoun that.
Debpriya De"You signify all that I hate.I agree with Wordy that "signify" is not a good verb here, although the meaning is understood.