Since "foggy dew" scans as well as "foggy, foggy dew," I suspect the extra "foggy" is a later development.

The phrase may have been suggested by the Irish tune "The Foggy Dew."

Interestingly enough, a bay gelding of that name ran at the Perth Races on Oct. 7, 1814.

That's the earliest appearance of the "foggy dew" I know of.