1948 Nickle Is Error?

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A doubled die is made when two images of the same design are engraved into the die. You end up with a doubled image. There is another type of doubling that is common called Machine Doubling (MD). Learning the difference just takes a bit of reading here on the forum concerning this subject. There are many threads to look through.

Machine Doubling is when the dies tha coin are stamped from are slightly loose, and so they bounce when striking the coin. Hence the design can be "smeared" and you get a flat, shelf-like doubling.

For legitimate doubled dies, the device will be made thicker in appearance than normal since there are two images engraved side by side. For MD, the devices will be made thinner. So, for example, a machine doubled 1 in a 19XX date will have a shelf-like image of the 1 beside it while the original 1 is made thinner than normal.

If you look at varietyvista.com, you can find known doubled dies for every denomination and year. Note some of these are very difficult to see - as with the 1948 nickel:
http://varietyvista.com/04a%20JN%20...8PDDO001.htm

In some of these pictures you can see a very thin doubling to the side of some of the letters/numbers. Generally a doubled die that is this slight is not worth a premium.

And what makes you think it's a Doubled Die? I can tell you that it's definitely not even close. A normal coin.

The devices as the coin wears down in circulation makes the devices look larger. On a doubled die with the same wear, the devices would show even larger. I feel your coin is normal. The die wear on the devices is consistent on all of them. The devices on a doubled die could have some more stronger doubled than others. I feel it is a normal worn coin.

Well it should be raised if it is a die crack. But with the coin wear, then it may have worn the raised area off the coin. A closer view of might help us a bit more. But if the area is incuse (sunk into the coin) then it might be damage or a struck through issue. I see the mark on the jaw area, but don't see anything on the upper part of the head?