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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 canPrepare(self, ppReq) -> (bool, str):
 """ Return whether the batch of requests in the PRE-PREPARE can proceed to the PREPARE step. :param ppReq: any object with identifier and requestId attributes """

        if self.has_sent_prepare(ppReq):
            return False, 'has already sent PREPARE for {}'.format(ppReq)
        return True, ''