I'm not sure if anyone besides Austin P gets what I was saying, why I reopened this thread (in which many of the ideas in recent posts were already covered). So I'll try to state in brief one more time, then I'll shut up ;)

As I said before, I think "dou dou", "do do", or anything else we might imagine is misheard is a red herring. As Q is now iterating, there is no strong reason to think the "doodle" means anything besides "do" or "doodle." In fact, why would Hugill not get it if it was "dou dou," a term he was very familiar with?
"Oh once I met a dou dou fair, belong to Mobile Bay
Hooraw, me yeller gals, doodle let me go." There, he just said it 10 words before!

Incidentally, "doodle let" doesn't sound exactly like "do do let." The quality of the second is different, like in "yodel"...when yodeling....

...which is very similar to what I argue is happening:

"doodle let" = "do let" ((the alternate form given by Hugill (via Harding) and Sharp (via Short)) with a particular, optional, customary style of ornamentation inserted.

I'd given evidence of this above, by way of other examples of this occurrence, all in the same phonemic environment, along with reasoning from the textual evidence. The response was (sorry for the rough paraphrase):

--Tom saying "Try singing 'do, do let'" OK, I did. Now what? Waiting for the Zen moment...
--Barry quoting chanteys with the term "dou dou" -- proving only that such a thing as "dou dou" exists in the world...
--Azizi researching the etymology of "dou dou," I think mainly because she is secretly a 19th century German philologist...

I agree with Q that the doodle "fills out the line." Where I differ is that I'm suggesting it is a meaningful word ("do") combined with such ADDED baggage to fill out the line/rhythm/melisma.

Gibb