“a gzera on a gzera” –
there are three types of gzeros:
1. gzeros to prevent issur d’oratsa.
2. gzeros that Rabbonon made that are independent of aany other reason.
3. gzeros that are part of an issor min hatorah (such as doing a melacha on shabbos – sh’aino tzricha l’gufa).