I received an email with attachments. The text opened with "Voili-voilou les notes de..." Is voili-voilou known argot for voilà? Or something else?
Yes, it is kind of a jocular alternative for voilà, or voilà voilà !
It is not very used and has no inherent particular meaning, but is definitely understood.
The tone used will tell how to interpret it, but the rare times I heard it, that was to express the happiness to have completed something and to deliver it like "Yeah, here you go!" or to mark that you have finished to tell something like disclosing some story, and soften the end with a "That's all, folks!".
"Voili Voilou" is quite common and used much more frequently verbally rather than in writing. It is only used in very informal situations such as at home or with close colleagues. Compared with just "Voilà" there is a note of finality. "Voilà" when handing a report written by someone else a close colleague and you are just passing it on, "Voili Voilou" when you hand a report that you wrote yourself over the last week. As with all subtleties, there is no golden rule