avoid adding same file more than once, resolves #16

Signed-off-by: BlackEagle <ike.devolder@gmail.com>
enhance handling of duplicate files on verify/repair

- When a duplicate source file was detected, the program just exited
- The verification of the files can take place even if the parfiles
  created are 'corrupt' due to the adding of the same sourcefile
  multiple times.
- Repair seems to be a very hard task to achieve when 2 files have the
  exact same filename

- solution, allow nice verification, repair will not work

Signed-off-by: BlackEagle <ike.devolder@gmail.com>