Guard against multiple calls to Logger, Webserver

[wandernauta]

The configuration parser allows something like this:

Webserver{address="192.0.2.10:8080", user="api-access-for-robot", password="robot"}
Webserver{address="192.0.2.20:8090", user="dashboard-for-human", password="human"}

But the behavior is potentially unexpected:

• For Logger: the "last one wins".
• For Webserver: "both win" (you indeed get two endpoints) except that the username and password for both endpoints are taken from the last call.

This makes it so that any second call to Webserver or Logger is rejected.