In How old is the reduplicated perfect in Indo-European? I was attempting to get a handle on a reasonable dating of the reduplicated perfect in Proto-Indo-European (PIE) within the context of my grandiose theories on Pre-IE. As a recap, the reduplicated perfect form is demonstrated by the form *bʰe-bʰor-e meaning 'he/she has carried' where we can see that the first sound of the root *bʰer- is repeated and the root is given the vocalism *o. which is then followed by the personal ending *-e for the third person singular. Additionally, the perfect forms have a completely different set of personal endings from the present forms. So, we need to figure out how this might have come about. I previously suggested that i-reduplicated presents (e.g. *di-deh₃-ti 'he/she gives') were older than the reduplicated perfects but after meditating on this the whole day yesterday, I believe I've been missing the opportunity for more efficient solution.
So instead, I would like to explore the possibility that it is to the contrary the reduplicated perfect that dates to the Mid IE (MIE) stage before the Syncope rule while i-reduplication seen in the presentive forms is a much later feature of Indo-European conjugation. The reason for my change will become clear as I walk you through the proposed evolution. I already have common examples like the genitive-declined form *pedós 'of the foot' which contrasts with nominative *pōds showing *o/*e ablaut alternation that can be attributed to the change of unsyncopated unaccented MIE *a in the root to early Late IE (eLIE) *e in order to avoid making declension too obscure with overly erratic root vowel deletions in the paradigm of a noun (i.e. the Paradigmatic Resistance exception of Syncope).
Given that, it just makes perfect sense to exploit this preestablished exception (i.e. MIE unaccented *a > *e rather than deletion) to explain the appearance of *e in the reduplication of the perfect. We may then simply reconstruct an MIE pattern *Ca-CáC- as the antecedent to the later so-called reduplicated perfect of the form *Ce-CoC-. in much the same way as MIE *pad̰ása becomes eLIE *ped̰ás and later *pedós. On the other hand, i-reduplication can then be blamed on the schwa found in the middle of the Late IE period which would be a natural vowel to initially use when reduplicating a present form to convey the iteration of an action. Thus, eLIE *Cə-CéC- may serve as a more straight-forward antecedent to later *Ci-CeC- via my already-stated rule "preaccented eLIE *ə > PIE *i".
However, there's still the unresolved controversy concerning what the real function of the reduplicated perfect in PIE itself and its preceding stages was exactly if it were ancient. For this, I feel the need to appeal to the tenseless language I'm most familiar with, Mandarin. I will suggest briefly that perhaps there was originally a simple two-fold distinction between 'non-completed' actions in e-grade and 'completed' actions in a-grade (later o-grade due to Vowel Shift at the end of the Late IE period). This grammatical structure would find a parallel then in Mandarin where completed actions are marked with the particle 了 le placed after the verb as in 我看了 wo kan le 'I looked'. There is also a further marker in Mandarin for the progressive, expressed when 在 zai or 正在 zhengzai is placed before the verb as in the example 我在看 wo zai kan 'I'm watching/reading'. It must be stressed that the completive is not the same thing as past tense since there is the example of (现在)下雨了 (xianzai) xia yu le meaning 'it is raining (now)' (i.e. 'it has finished starting to rain (now)', so to speak) whereby le appears in effect to be marking an inchoative action. Confused? Excellent!
Back to PIE then, perhaps likewise in MIE there was once a "completive" aspect using the form *CaC- together with the *h₂e-set of personal endings distinct from the *mi-set. Then a reduplicated form developed out of this in MIE (*Ca-CáC-) to express an action that was continuative at some point but was thereafter completed. So perhaps a simple form in MIE like *bár-a came to mean "he/she has carried (once)" while *ba-bár-a meant "he/she was carrying (but now he/she is done)" which would oppose the durative past *bér-ata "he/she was carrying (and may still be carrying)." The two meanings between the simple a-grade and the reduplicated a-grade would only be minutely different but the latter form would be barred for inherently stative verbs like *waid̰- 'to know' which could only be used in the form *wáid̰-a meaning "he/she knows" (or possibly with inchoative meaning "he/she came to know") but never with continuative aspect as in **wa-wáid̰-a "he/she was knowing (but now he/she is done)." If you think about it, the reduplicated form could easily lend a resultative nuance if analysed in this way since the reduplication would have originally stressed the non-stative quality of the verb (either "repetitive" in nature as for punctual actions, or "continuative" as for non-momentaneous ones) while the *h₂e-set of personal endings would ensure a completive aspect in contrast to the non-completive *mi-set. In other words, the reduplicated a-grade would both stress an action rather than state as well as its eventual endpoint. Thus a natural rift would grow over time between the inflection of resultative verbs and of stative verbs within the originally united category of completive aspect. Additionally, the reduplicated a-grade might still have avoided crystallizing into a formal perfective as long as non-reduplicated forms of active verbs still had a function within this system.
So could this mean that *bʰor-e continued to mean "he/she has carried (once)" in PIE while *bʰe-bʰor-e meant "he/she was carrying (but now he/she is done)" until the Anatolian branch seperated from the Indo-European core? Does this mean that the simple CoC- construct remained the preferred resultative-stative form until after PIE? Is the mi-class/hi-class division found in Hittite to be understood as a derivative of this aforementioned two-fold system whereby the inherited hi-class verbs are largely either punctual or stative in nature?
 Soh/Gao, Perfective Aspect and Transition in Mandarin (2006), University of Minnesota, p.111 (see pdf).