I am testing some problems with xboard and mayhemduck under linux:
https://talkchess.com/forum3/viewtopic.php?f=7&t=80829&start=180

Here is one of the games I played against mayhemduck18:

1.e4,e6 Nf6, c3
2.e5,d4 Ng4,f3
3.d4,d6 Nc6,e2
4.Nf3,d6 e6,f5
5.h3,h6 Bb4,a4+

My intended move was 6.hxg4,c3 but this was incorrectly flagged as illegal. There is no such thing as "check" for Duck chess.
I just played 6.c3,c5 and eventually won both the Knight and Bishop.