What causes letters to have a shaddow?

I couldnt get a great picture of it but all the letters have a shaddow. What causes this?

 

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Usually a Deteriorated die. The US Mint is known for using their dies well beyond the point of needing replacement.

 

Thank you i google what that was

 

Or you can go to www.doubleddie.com and www.error-ref.com
There are other sites as well. Clearer photos would also be helpful.