Is there a name for kanji compounds consisting of kanji with the same radical and for compounds with repeated kanji?

Question

There are lots of 二字熟語, say, where both characters have the same radical, e.g., 明日, 機械, and 往復. I find these interesting as often the common radical is strongly associated with the meaning of the compound. Of course, there are also lots of compounds where the character itself is repeated（e.g. 色々, 数々, and 日々), though I would put these in a different category.

Is there a name for either of these types of kanji compounds?

I tried looking at the Wikipedia entry for 踊り字 for the second type, but didn't notice any special word for it used there, just something like 同じ漢字を重ねる.

Answer 1

We don't have specific words for kanji or radical reduplication, bacause it's merely coincidence, or a side effect happens when we put related or the same words together.

As for repeating the same words, we call it 畳語. If a word that written in a single kanji is repeated, it appears as if the kanji is repeated. But if you try with other words, you'll get 場面{ばめん}場面{ばめん} (or 場面々々) "situation by situation", 知{し}らず知{し}らず "unconsciously" or ぴかぴか "glitter-glitter", too.

Categorizing words whose components have the same radical is generally not as good an idea as it seems to be. I won't deny its use for mnemonics, since radicals with strong semantic restriction (like 木, 虫 or 魚) may create more same-radical (dvandva-type) compounds, but as a whole, we don't think coincidence of radical itself is associated with some essential nature of our writing or language. Radicals often only shed light on their meanings from one side; 機械 both have radical Tree, but today's people easily notice that it's starting to deviate from what it's supposed to mean. How 消滅{しょうめつ} "vanish" is related to Water^(*1), and how 陰険{いんけん} "sly and mean" to Cliff^(*2)? Actually, they're examples two characters of one radical happened to be brought together, irrelevant of what the radical meant. 明日 is more like false positive; 明's 日 "sun" is historical simplification of 囧 "window". A similar case is 鮮魚{せんぎょ}^(*3) "raw fish". Although these words share the same radical, you'll see graphically unrelated 鮮明{せんめい} "vivid" shows far much semantic relevancy with each other.

There is, however, a small number of words that yield same-radical compounds with high probability. 葡萄{ぶどう}, 混沌{こんとん}, 齟齬{そご} and 魑魅魍魎{ちみもうりょう} all belong to 連綿語. For instance, 葡萄 "grape" both have Grass, because they put it intentionally to make sure it refers to a plant. (Neither 葡 nor 萄 is used outside "grape" or transcription of Portugal, 葡萄牙.) These words had their sounds in multiple syllables first, then got their kanjis later. Basically it's a matter of Chinese that affecting Japanese writing too.

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*1: 消 originally meant "rivers run dry", and 滅 meant "annihilate" in its bare form 烕. Why 滅 has got Water is not clear, but probably was association with its another meaning: "extinguish fire".

*2: 陰 means "overcast, dark". It got Cliff while heavily used to mark place names in mountains' shade (north) side. 険's one just came from one of its meaning: "precipitous pass".

*3: 鮮 originally meant a name of fish. Using it for "fresh, lively" is a kind of rebus.

Answer 2

It's 畳語(じょうご) / reduplication, though it's not limited to kanji.