Finally got some time to find a way around the problem, that each character was printed out twice when connecting to the MBC2 by telnet. What worked for me was to use PuTTY with some changed settings:

== /Session ==
* Connection type: Raw

== /Terminal ==
* Local echo: Force off
* Local line editing:  Force off

== /Terminal/Keyboard ==
* The Backspace Key: Control-H

That way i could finally try the Ladder game that was included, and even Groilla works with color output when selecting ANSI-Terminal. I’m glad that I finally figured that out :)