A reduplicative is a word or lexeme (such as mama) that contains two identical or very similar parts. Words such as these are also called tautonyms. The morphological and phonological process of forming a compound word by repeating all or part of it is known as reduplication. The repeated element is called a reduplicant.
David Crystal wrote in the second edition of The Cambridge Encyclopedia of the English Language:
"Items with identical spoken constituents, such as goody-goody and din-din, are rare. What is normal is for a single vowel or consonant to change between the first constituent and the second, such as see-saw and walkie-talkie.
"Reduplicatives are used in a variety of ways. Some simply imitate sounds: ding-dong, bow-wow. Some suggest alternative movements: flip-flop, ping-pong. Some are disparaging: dilly-dally, wishy-washy. And some intensify meaning: teeny-weeny, tip-top. Reduplication is not a major means of creating lexemes in English, but it is perhaps the most unusual one."
(Cambridge Univ. Press, 2003)
Reduplicatives can rhyme but aren't required to. They likely have a figure of sound represented in them, as alliteration (repetition of consonants) and assonance (repetition of vowel sounds) would be common in a word or phrase that doesn't change much among its parts, such as in this by Patrick B. Oliphant, "Correct me if I'm wrong: the gizmo is connected to the flingflang connected to the watzis, watzis connected to the doo-dad connected to the ding dong.”
According to "Gift of the Gob: Morsels of English Language History" by Kate Burridge:
"The majority of… reduplicated forms involve a play on the rhyme of words. The result can be a combination of two existing words, like flower-power and culture-vulture, but more usually one of the elements is meaningless, as in superduper, or both, as in namby-pamby. Now, it struck me the other day that a large number of these nonsense jingles begin with 'h.' Think of hoity-toity, higgledy-piggledy, hanky-panky, hokey-pokey, hob-nob, heebie-jeebies, hocus-pocus, hugger-mugger, hurly-burly, hodge-podge, hurdy-gurdy, hubbub, hullabaloo, harumscarum, helter-skelter, hurry-scurry, hooley-dooley and don't forget Humpty Dumpty. And these are just a few!"
(HarperCollins Australia, 2011)
Reduplicatives differ from echo words in that there are fewer rules in forming reduplicatives.
The history of reduplicatives in English starts in the Early Modern English (EMnE) era, which was about the end of the 15th century. In the third edition of "A Biography of the English Language," C.M. Millward and Mary Hayes noted:
"Reduplicated words do not appear at all until the EMnE period. When they do appear, they are usually direct borrowings from some other language, such as Portuguese dodo (1628), Spanish grugru (1796) and motmot (1651), French haha 'ditch' (1712), and Maori kaka (1774). Even the nursery words mama and papa were borrowed from French in the 17th century. So-so is probably the sole native formation from the EMnE period; it is first recorded in 1530."
Morphological and Phonological
Sharon Inkelas wrote in "Studies on Reduplication" that there are two separate methods, producing two different types or subsets of reduplication: phonological duplication and morphological reduplication. "Below we list some criteria for determining when a copying effect is reduplication and when it is phonological duplication.
(1) Phonological duplication serves a phonological purpose; morphological reduplication serves a morphological process (either by being a word-formation process itself or by enabling another word-formation process to take place… ).
(2) Phonological duplication involves a single phonological segment… ; morphological reduplication involves an entire morphological constituent (affix, root, stem, word), potentially truncated to a prosodic constituent (mora, syllable, foot).
(3) Phonological duplication involves, by definition, phonological identity, while morphological reduplication involves semantic, not necessarily phonological, identity.
(4) Phonological duplication is local (a copied consonant is a copy of the closest consonant, for example), while morphological reduplication is not necessarily local." ("Morphological Doubling Theory: Evidence for Morphological Doubling in Reduplication." ed. by Bernhard Hurch. Walter de Gruyter, 2005)