Semantics of /ud-/ vs/ /u.d.d.-/

Although this is hardly "critical", I couldn't find anything of use in
the online CPD re:

/ud-/ vs. /u.d.d-/

I'm guessing that there's an etymologically unrelated form that
sometimes appears as /u.d.d-/, lacking any real semantic link to /ud-/
(viz., despite the fact that the later could "permute" to a retroflex,
reduplicate, etc.)

It would be interesting to know more about /ud-/ vs. /u-/1 vs. /u-/2
(upasagga) --the latter, I take it, from an unrelated (vedic) /ud-/
that we now have more commonly as /uttara-/, etc.?

E.M.