Don't suppress other errors if a parameter wrongly appears multiple times

[cbeck88]

If the program is used as ./my_prog --param foo --param bar, when param is a Parameter and not a Repeat field, the clap parser fails fast because we tell it to only expect this parameter to appear once.

Instead, these two parameter types should look the same from clap's point of view, so that if it appears multiple times, we can report other missing or invalid arguments as well.