Cannot override default net device

[aidalgol]

Trying to use the example config vm-forward.nix produces this error:

qemu-kvm: -netdev user,id=user.0,hostfwd=tcp::2222-:22: Duplicate ID 'user.0' for netdev

This is on a nixpkgs-unstable system. I can use QEMU_NET_OPTS as a workaround.