it checks if this id is already registered ?

from pysnmp/carrier/base.py:

    def registerRecvCbFun(self, recvCb, recvId=None):
        if recvId in self.__recvCallables:
             raise error.CarrierError(
                'Receive callback %r already registered' % (recvId is None and '<default>' or recvId,)
            )
        self.__recvCallables[recvId] = recvCb