How to fix duplicate symbol error in compiling simple module inheriting INET EtherTrafGen class in omnet++?

I try to make my own traffic generator module inheriting INET EtherTrafGen in omnet++, only resulting in duplicate symbol error. I can't figure out the reason. Below are the compile error messages.

..
Creating executable: ../out/clang-release/src/hdc.exe
lld-link: error: duplicate symbol: inet::OperationalMixin<omnetpp::cSimpleModule>::isUp() const
>>> defined at ../out/clang-release/src/TrafGen.o
>>> defined at libINET.dll

lld-link: error: duplicate symbol: inet::OperationalMixin<omnetpp::cSimpleModule>::isDown() const
>>> defined at ../out/clang-release/src/TrafGen.o
>>> defined at libINET.dll
clang++: error: linker command failed with exit code 1 (use -v to see invocation

..

The corresponding header file, i.e. TrafGen.h is

    #include "inet/applications/ethernet/EtherTrafGen.h"

namespace inet {


//class TrafGen : public cSimpleModule ----> In this case, no compile error
class TrafGen : public EtherTrafGen
{

protected:
    virtual void initialize() override;
    virtual void handleMessage(cMessage *msg) override;

};

}

and the corresponding TrafGen.cc file is

#include "TrafGen.h"

namespace inet {


Define_Module(TrafGen);

void TrafGen::initialize()
{
    if (strcmp("hostA", getName()) == 0) {
            cMessage *msg = new cMessage("hdcMsg");
            send(msg, "out");
    }
}


void TrafGen::handleMessage(cMessage *msg)
{

    send(msg, "out"); // send out the message

}

}

TrafGen.ned is

import inet.applications.ethernet.EtherTrafGen;

simple TrafGen extends EtherTrafGen
{
    parameters:
        @class(inet::TrafGen);

}

By the way, when the TrafGen: public 'cSimpleModule' is used instead of 'EtherTrafGen' in the definition of TrafGen in the TrafGen.h, no compile error (duplicate symbol error) occurs.