The present is made by adding the personal endings directly to the verb-stem, which is a root. This verb-stem may be used in its pure form or it may be reduplicated.
a. Some verbs of this class with no active have a verb-stem of more than one syllable (usually two syllables).724
Unreduplicated Presents: εἰμί (ἐς-) am, εἶμι (ἰ-, εἰ-) go, ἧμαι (ἡς-)
Deponents.—ἄγα-μαι (and ἀγάομαι) admire, δέα-μαι
a. Other such forms are Hom. ἵ_εμαι (ϝί_εμαι)
a. Also poetic βίβημι (βα-, βη-)
Verbs in -μι reduplicate with ι in the present. See cross414, cross447. πί-μ-πλημι and πί-μ-πρημι may lose the inserted nasal in compounds of ἐν, but only when ἐν- takes the form ἐμ-; as ἐμπίπλημι, but ἐνεπίμπλασαν. Doric has κίγχημι. In ὀ-νί-νη-μι the reduplication takes place after a vowel (verb-stem ὀνα-, ὀνη-).
a. Reduplication is in place only in present and imperfect; but Hom. has διδώσομεν.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].