Currently we exit when trying to add a duplicated domain, even if we give a list
of domain names. This behaviour is not as it suppose to work.
**Expected behaviour**
When adding a domain, and the domain already exists, just skip that domain, and
continue with the world. This is no problem since we are already in the state we
want to be in.
**New behaviour**
When you try to add a domain that is already added, we notify the user of this
by showing a log message: `Skipping: example.com already added to example app`,
and then we continue with the list of domain names.
Fixes#2165
4f621916a7