>compare Sanskrit barbara- "stammering,"

Interesting. Though I knew Sanskrit (well in school), didn't know or remember that word - but still know Hindi well, and in Hindi, bad-bad (pronounced like bud-bud) means blabbing, as in:

Kya bad-bad kar rahe ho - What are you blabbing.

And to close the loop, in Hindi, the sounds r and d are closely related and sometimes substituted for one another - at least while talking.