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.
Note flatness of the doubling on the left, reason for the "shelf" description.
A double punched digit will show a more rounded outline.
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).

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.