Yes the US keeps printing new bills but when they do they replace old bills which is why we don't see old 20's and other bills. When bills are printed new and stores turn in money it is just recorded and weeded out. The difference of the US and Iraq is that Iraq just kept printing without taking any out of circulation.
If the US prints a new 20 they will destroy an old 20. I will say that back in March of 2009 Bernanke announced we will dilute the dollar causing a 700 point jump on the dollar getting shorted on the Forex.
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