Martin Sheen (Reuters/Mario Azuoni)

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Despite season nine episode 'The Principal and the Pauper', where it is revealed that Principal Skinner is an imposter named Armin Tamzarian, being much-maligned in terms of what it does for the continuity of the show, it nonetheless features a fantastic guest appearance by the great Martin Sheen as the man revealed to be the real Seymour Skinner.