Reduplication

Reduplication is the process of repeating a word or part of it to express some grammatical function, plurality for example. It is used in some language families' grammar, most notably in Malayo-Polynesian where it forms plurals:

Bahasa Malay rumah, house, rumah-rumah, houses.

Hawaiian has the important example of Wiki-wiki!

Nama uses reduplication to increase the force of a verb:

Go, “look”, Go-go, “examine with attention”.

Chinese also uses reduplication:

ren, “person”, renren, “everybody”

Indo-European languages formerly used reduplication to form a number of verb forms, especially in the preterite or perfect tenses. In the older Indo-European languages, many such verbs survive:

spondeo, spopondi (Latin, “I vow, I vowed”)
λείπω, λέλοιπα (Greek, “I am missing, I was missing”)
háitan, haíháit (Gothic, “to name, I named”)

None of these sorts of forms survive in modern English, although they existed in its parent Germanic languages.