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1.
a sex act similar to fellatio but when the man takes control by forcibly moving his penis with a high velocity in and out of the mouth/throat so as to make a "uck-uck" sound in the process. Hence the term duck duck.

Note: can be used as either a noun or a verb.
He took her round the corner for a quick duck duck.
by mwahaha1234 May 24, 2012
 
2.
To fail at something over a period of time. Usually followed by numbers to show others how long they have failed at something for.

Can also be used in conjunction with words such as fuck, suck, dick, dyke.
Dude, that guys been Duckducking since 2003!
by Connor Lockwood October 09, 2008