Here is an example of shelf doubling, or strike doubling. This is caused by the die moving just after the strike,
It does not add value to the coin.
MNS_1857_shelf_1_Oct10.jpg
Note flatness of the doubling on the left, reason for the "shelf" description.
A double punched digit will show a more rounded outline.
MNS_1857_shelf_8_Oct10.jpg
MNS_1857_shelf_5_Oct10.jpg
Note weakness of 5. This is caused by a debris strike-through. Debris was stuck in the die and filled in thew detail when the coin was struck. Debris could be grease, water, or a workman's thumb (hopefully not).

MNS_1857_shelf_T_Oct10.jpg
The strike doubling is also on the letters. Notice that until 1909 the date and legend were added to the die as separate times.

If you have the same doubling on both the date and legend it is likely strike doubling.