I know that classifiers, verbs, and adjectives can be duplicated. However, the expression 五六十年代年代 seems to contain a duplicated noun instead. The meaning is clearly "the decades of the 50s and the 60s", but what's the grammar behind this construction? Google does not seem to find any further instance of 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:
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:
I doubt the translators intent was to patronize a young one.