Doubling On Serial Number

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I received this in change this morning. On the right hand serial number there's "doubling" on the last letter of the prefix and the first four digits of the number. I've seen this on single digits but not on so many. How common is this?

I just found one almost exactly like it, so I'm venturing to guess this is not uncommon.

I just came across a range of new $20 notes that all had the "doubling" on the right serial number.  In fact, you could say the doubling gives the digits a 3-dimensional look.  It's just extra ink that is smearing and creating the minor error.

{http://www.give-a-buck.com/special/arr-ghosting.jpg}

The right hand serial number has "doubling" but in addition there is a ghost "0" behind the third last digit.