Machine Doubling
Machine Doubling (also called mechanical doubling, shift doubling, or strike doubling) is very different from Die
.  Machine doubling is created when the die shifts slightly as a coin is being struck.  Machine doubling is
the result of loose mechanical parts that cause a poor strike.  In
machine doubling there is no doubling in the
die, just in the strike.  In a
Doubled Die, the die itself contains the abnormal doubling.   Die doubled coins are
considered varieties, and are often valuable.  
Machine doubled coins are considered errors, and are not of any
significant value to collectors.

In machine doubling, the secondary image is often a flat shelf-like surface.  In Die Doubling, the
secondary image is rounded and looks similar to the primary image.  A tell tale sign of
machine doubling is when
the mintmark and date show the same doubling.  

I see machine doubled coins sold on Ebay almost every day.  Below are some pictures.  My advice is to not
purchase any doubled die coins that you have not seen pictures of in a book or website.  
The Cherrypicker's
Guide to Die Varieties
gives detailed descriptions of how different types of doubling are created.  

The following are Machine Doubled Lincoln Cents that have been listed on Ebay as "Doubled Dies":