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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def echo_worker(self):
 """ The `echo_worker` works through the `self.received_transfers` queue and spawns `self.on_transfer` greenlets for all not-yet-seen transfers. """

        log.debug('echo worker', qsize=self.received_transfers.qsize())
        while self.stop_signal is None:
            if self.received_transfers.qsize() > 0:
                transfer = self.received_transfers.get()
                if transfer in self.seen_transfers:
                    log.debug(
                        'duplicate transfer ignored',
                        initiator=pex(transfer.initiator),
                        amount=transfer.amount,
                        identifier=transfer.identifier,
                    )
                else:
                    self.seen_transfers.append(transfer)
                    self.greenlets.add(gevent.spawn(self.on_transfer, transfer))
            else:
                gevent.sleep(.5)