Reduplication in 五六十年代年代
It’s just a typo.

We have the, original, English for comparison:


  This energy-efficient mode of long-distance rail connectivity stands in sharp contrast to the carbon-intensive US interstate highway system created for motor vehicles in the 1950s and 1960s.


Along with the Chinese translation:


  这种节能高效的长距离连通模式，同上世纪五六十年代年代美国州际公路系统，形成了鲜明的对比。


The original text contains zero hints of duplication for 年代. It seems to just be a straight up mistake.

It is also clearly marked as a translation on the Chinese page:


  Translated by Congyi Shi


So we know it's not original text from the writer themselves.



The only real time you’d see double nouns is when talking to a baby or an infant:


(吃)饭饭 
(喝)水水 


I doubt the translators intent was to patronize a young one.