The OP question was addressed and answered.
But the OP also assumes that we do not make a gezeira l’gzeira.
That is only PARTIALLY right… There are EXCEPTIONS!
It is important to set the record that the statement ” Isn’t that a gzera on a gzera?” implyingthat we cannot and never make a gzera on a gzera – is wrong!
The following is a PARTIAl list of exceptions:
Beitza 3a the gemara itself (after statingthat we don’t make a gezeira l’gezaira on 2b) states that if it is one inclusive gezaira, they we do “expand” the gezeira. This exception “would” (if not for the correct answer) also apply to kitniyos on erev pessach, that the gezaira was expanded to include all times that chometz is forbidden to be eaten, both min hatorah and middrabbonon.
Tosfos in Brochos 531 adds another exception, that if the first gezaira wouldn’t be kept without the secondary gezaira, they we do add the second gezaira l’gezaira. Again this would also apply, if one could eeat kiutniyos at times of issur, one would continue to use them.
Meiri on Beitza 37a gives another exception when the gezeira has support from a possuk.
Tosfos Rid Shabbos 31a gives further exception if the gezeira is an independent one, not related to an issur from Torah, then further gezeira can be added.
Chacham Tzvi s. 75 says that when a common issue that happens often, a gezera l’gezaira can be added.
Similar, GRA on O.Ch. s. 252 states that in cases of negligence, we do make gezeira l’gezaira.
(A little knowledge leads to grave misunderstandings of the knowledge).