e/o

As I understand it, there is agreement that at an early stage of IE,
the vowel /e/ arose in stressed syllables, and /o/ in unstressed ones
(and then some more etc). This is a phonetic explanation. But then
there is the -o- of the pret. sing. stem, part of the ablaut pattern.
This -o- seems to be the conditioned by inflection. Confusion.

The older the IE language, the more reduplication it has in verbal
inflection. So I wondered, suppose reduplication was once mandatory,
then you would have forms like CéCoCC (assuming that the (pre?)
reduplicated syllable was stressed). Yes?

Sorry for the sloppy formulation.

Torsten