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The first occurrence of the multiple repeated knocks appears in Series 1 Episode 10 - The Loobenfeld Decay:

Sheldon: (Knock, knock, knock, knock) Leonard, (Knock, knock, knock, knock) Leonard, (Knock, knock, knock, knock) Leonard...

Sheldon: (Knock, knock, knock, knock) Penny, (Knock, knock, knock, knock) Penny, (Knock, knock, knock, knock)Penny, (door opens) Good morning.

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9y ago
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13y ago

Season 3, Episode 22: The Staircase Implementation

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Q: On which episode of The Big Bang Theory did Sheldon first make his signature knock on Penny's door?
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