In case you don't know what it is, here's the definition:
[quote]Favorite behavior of crude diners. Involves dipping your crudite or corn chip into a sauce, taking a bite from the veggie or chip, and then re-dipping the half digested item back into the sauce
Made famous on Seinfeld some years back.
Sign at country fair food stall: "DO NOT DOUBLE DIP!"[/quote]
Source: Urban Dictionary
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I think mythbuster covered this topic before