Options should not be able to be added more than once

One should not be able to add the same Option (by unique ID) to the Options
object multiple times. Specifically, this causes a problem with required options.

I performed the following set of steps, which resulted in the behavior that the
option I specified was not marked as set in the Parser.requiredOptions List:

Option o = OptionBuilder.isRequired().withDescription("test").create("test");
Options opts = new Options();
opts.addOption(o);
opts.addOption(o);

Now, if I parse a command line like this:

-test

I get an MissingArgumentException thrown from Parser.checkRequiredOptions that
the option was not found. This is because the option has been added to
requiredOptions twice.

I believe that one should not be able to add the same option twice (or this
behavior should be ... er .... optional).