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Just as the question proposes, I'm having trouble with proving this tautology. I know one should use proof by contradiction however I am currently stuck.

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    Assume (P↔¬P) and derive a contradiction. – Mauro ALLEGRANZA Mar 5 '18 at 7:07
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    Second step: unpack P↔¬P with -elimination rule. – Mauro ALLEGRANZA Mar 5 '18 at 10:13
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Here is a proof using the Fitch software:

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