fix(server/player): prevent login while already logged in

[solareon]

Description

We don't check if players are already logged in before letting them log in. This is bad form and lets folks dupe starter items and cause other issues. This checks if players are logged in and drops them for exploiting and triggers a log message.

Checklist

• I have personally loaded this code into an updated Qbox project and checked all of its functionality.
• My pull request fits the contribution guidelines & code conventions.