It's called a mint mark and identifies the mint that struck the coin. See the Related Question for more information.
Plain what? If you're referring to U.S. coins there were 5 denominations struck that year so more information is needed. The best thing to do is to post a new question with the coin's denomination and condition.
I modified your question because coins are struck or minted rather than "printed", which refers to books and banknotes. It's not possible to say exactly what happened without looking at your coin but my first guess is that it was exposed to some corrosive or acidic liquid. Because of the way copper "flows" when a coin is struck, the design will still appear clearly even when much of the metal has been dissolved. Unfortunately that makes it a damaged coin with no extra numismatic value. It's still an interesting curiosity, though.
You'll need to provide more information. What kind of double-strike is involved? The same image twice on each side, but out of alignment? Opposite images on each side? Or on one side only?
According to the US Mint, a total of 86,640,000 nickels and 146,000,000 dimes were struck for circulation in 2009. Those numbers were much lower than average due to the so-called Great Recession.
When the clock struck a new hour, the protagonist heard a mysterious sound like a gong coming from the mansion. This sound seemed to signal a change within the mansion, instigating curiosity and setting the scene for further intrigue.
It sank.
No. Struck would be a verb. It is past tense, but it isn't an idea, it is an action that happened.
A thought struck him
A thought struck him
The Titanic struck an iceberg and sank
a earthquake struck and killed Lots of children and adults
That didn't happen.
Metorites have crashed onto the moon which has not happened to the earth. Although the earth has been struck it has not happened enough to be noticable.
She dies like finnick
I expect that they struck the set, as they usually do after the last performance.
it was struck by lighning