Building the mono/tests/testing_gac/app-both.cs example with MCS results in the following error:
$ MONO_PATH=~/work/mono/mcs/class/lib/build ~/work/mono/runtime/mono-wrapper ~/work/mono/mcs/class/lib/build/mcs.exe -noconfig -nologo -debug:portable -target:library -target:exe -out:app-both.exe -r:V1=signed_v1/gactestlib.dll /r:V2=signed_v2/gactestlib.dll app-both.cs
error CS1703: An assembly `gactestlib' with the same identity has already been imported. Consider removing one of the references
signed_v1/gactestlib.dll (Location of the symbol related to previous error)
signed_v2/gactestlib.dll (Location of the symbol related to previous error)
Compilation failed: 1 error(s), 0 warnings
Note that signed_v1/gactestlib.dll and signed_v2/gactestlib.dll both have the same name and public key token but different versions (18.104.22.168 and 22.214.171.124, respectively).
Roslyn is able to compile the above example successfully.
Fixed in master