Django REST Framework: overriding get_queryset() sometimes returns a doubled queryset

I made a little endpoint, adapting DRF ReadOnlyModelViewSet, defined as follows:

class MyApi(viewsets.ReadOnlyModelViewSet):

    queryset = []
    serializer_class = MySerializer

    def get_queryset(self):
        print 'Debug: I am starting...\n\n\n\n'
        # do a lot of things filtering data from Django models by some information on neo4j and saving data in the queryset...
        return self.queryset

When I call MyApi via URL, it returns results without any problems, but sometimes it returns doubled result!! It's very strange...It's not a systematic error but happens only sometimes.

I use the line print 'Debug: I am starting...\n\n\n\n' in Apache log to investigate the problem. When that doubling happens, I read in the log:

Debug: I am starting...




Debug: I am starting...

It seems like get_queryset is called more than one time. It's very strange. I didn't report the detail of the logic inside that method, I think the problem is elsewhere or that is a bug...How can I solve?