What is the proper pronunciation of 等等？ Is 等等 pronounced:

deng2deng0 (2nd tone and neutral) like 老子、孔子

OR

deng3 deng0 (3rd tone and neutral) like 椅子、法子
It's pronounced

deng2deng3

as in the typical tonal sandhi of two consecutive 3rd tones, like 老板 (lao2ban3).
However I wouldn't outright exclude your first suggestion deng2deng0, as it might be pronounced like that in fast speech, or in the middle of a sentence.