fix #245: do not send mails to a person twice if he/she is delegate and editor.

fixes #245.

this only fixes the case that a person is editor of a course and delegate/cc-user of the responsible. there are numerous edge cases that are not fixed, e.g. if he is a delegate of another editor, but being delegate and editor should be the most common case, all other cases are rather theoretical i think.

the actual change is one line, but i've come a long way here and did a bit of refactoring before settling for this simple solution.