Here is and example of a good double print found on a cover with the "Vancouver 2010" overprint...Below are 2 of these covers.

Below is a normal overprint on this cover.

Below is the actual double print as you can see in the stem of the maple leaf.

At first I thought it might be a "slip print" or a "kiss print" but was not...Ralph's (re-entries.com) explanation of double prints are explained below....
Quote:
True double prints are quite a different matter, and in most cases, quite rare. A true double print is just that - printed twice, from a fully inked plate, receiving two complete, full impressions caused by passing through the printing press twice.
There are lots and lots of shifted prints in the Scott 1931 (NO Vancouver 2010 overprints)..This was a nice little find.
Robert