There were several types of Indo-European verbs formed by reduplication. The most important of them are listed below. Each type is represented by a verb in its 3sg form (of the active voice, where relevant); for glossing purposes, a female subject is assumed:
- athematic presents with a Ci or Ce echo: *sti-stéh₂-ti [*stistáh₂ti] ‘she’s rising to her feet’ ;
- thematic presents with a Ci echo: *sí-sd-e-ti [*sízdeti] ‘she’s taking a seat’;
- thematic aorists with a Ce echo: *wé-wkʷ-e-t [*wéukʷet] ‘quoth she’;
- perfects with a Ce echo: *me-món-e ‘she remembers’.
There are also a couple of other reduplicated present types, marked by the use of derivational suffixes. All of them have Ci echoes and are not very different from the second type above:
- reduplicated sḱe-presents: *dí-dḱ-sḱe-ti [*dítsḱeti] ‘she accepts/learns’;
- reduplicated desideratives: *wí-wrt-h₁se-ti [*wíwr̥tseti] ‘she wants to turn’.
A famous reduplication (almost too perfect to be true)
Still other types can be found in some languages of the family but cannot be safely added to the inventory of Proto-Indo-European verb stems because they are are either too poorly attested or too restricted in their distribution. The former is true of athematic reduplicated aorists, and the latter of the Indo-Iranian intensives with “full” reduplication (more precisely, with a CVC echo). Attempts to demonstrate their PIE status have not been successful so far.
I shall begin with the first two types (“underived” reduplicated presents, both athematic and thematic). I’ve already had to mention reduplicated presents in earlier posts. There is some kind of relationship between them and reduplicated nouns, and some of the same issues, like the *e ~ *i alternation in the echo syllable, will be revisited. The exact reconstruction of the reduplicated present is one of the hot problems of Indo-European morphology, not yet settled to everybody’s satisfaction, but important enough for people to keep trying. In the technical literature on the subject, you will find a variety of proposals which can’t all be correct at the same time. I don’t insist that the analysis I’m advocating is the solution; still, it’s more worthwhile to take the bull by the horns and tackle a vexing question than just to report handbook stuff. Controversy makes for an interesting debate.
One important special problem to be discussed separately is the reduplicated “present” stem  of the root *dʰeh₁- ‘put, place’ (plus a dozen or two other meanings it acquired in the early history of Indo-European). Next, I shall discuss the Indo-European perfect, partly because of its importance for understanding the origin of the Germanic “strong” past tense (English sang, drove, bound, etc.). The remaining loose threads will be tied up in the final post of this series.
 The reconstruction in square brackets is more phonetic, taking into account the operation of assimilatory processes, syllabification rules, and cluster simplification. The glosses are approximate: the exact shade of meaning produced by the combination of PIE tense, aspect and Aktionsart may be difficult to recover and even more difficult to convey in English.
 Why the scare quotes? Because the “present” (imperfective) stem did not occur only in the present tense, and it’s exactly the past-tense indicative of this stem, the so-called “imperfect” of PIE *dʰeh₁-, that played a role in the development of the Germanic verb system.