Add into the project file:

    agent exercise3c : participant.asl {
        roles: participant in agrp
               auctioneer  in agrp
    }

The execution produces the following error:

...
normative failure, details: fail(role_cardinality(auctioneer,"agrp",2,1))
    at npl.NPLInterpreter.verifyNorms(NPLInterpreter.java:250)

...

There could not be two auctioneers, as defined in the role carnality.