Duplicate maillist name/alias creation

Expected behaviour

When creating a mailing list or mailing list alias and trying to use an name that is already used, show an error message.

Actual behaviour

When creating a mailing list or mailing list alias and using an name that is already used, the list/alias is created. The pressing issue here is that mailing lists may accidentally be 'hijacked', because the mail is not send to both addresses.

Steps to reproduce

1. Create a mailing list mailtest
2. Make mailtest2 an alias of mailtest
3. Create a mailing list mailtest2
4. Observe there is no error while creating the alias and lists.

'Hijacking' side effect:

4. Send email to mailtest2@thalia.nu
5. Observe that no mail is sent to mailtest@thalia.nu