In 1999, mint marks were not hand punched into individual dies, but were a part of the working hub. If your coin did have a doubled mint mark, millions of others would have the same doubling. Yours is caused by machine doubling and/or a worn die.
5 cents. It isnt copper. It is environmental damage. Also, it is generally poor form to hijack someone elses thread. If you have a question about your coin you should create your own thread.