形容詞 dual:

1. consisting of or involving two parts or components usually in pairs
   同義詞：double, duple
2. having more than one decidedly dissimilar aspects or qualities-Frederick Harrison
   同義詞：double, twofold, two-fold, treble, threefold, three-fold
3. a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)

1. He has a dual interest in the football team; he's the trainer and his son plays for them.
   他跟那個足球隊(duì)有雙重關(guān)系，他是足球隊(duì)教練，他的兒子是隊(duì)員。
2. The road opened out into a dual carriageway.
   這條路已擴(kuò)展成了復(fù)式車行道。
3. His dual role as a composer and a conductor made him very busy.
   他作曲家和指揮家的雙重身份使他非常繁忙。
4. She has dual nationality.
   她有雙重國籍。

du.al

adj.（形容詞）
Composed of two usually like or complementary parts; double:
成雙的：由兩個通常相象或互補(bǔ)部分組成的；雙的：
例句：
dual controls for pilot and copilot; a car with dual exhaust pipes.
飛行員與副飛行員復(fù)式操縱裝置；有雙排氣管的汽車

Having a double character or purpose:
有雙重特點(diǎn)或目的的：
例句：
a belief in the dual nature of reality.
認(rèn)為現(xiàn)實(shí)具有二重本性的看法

Grammar Of, relating to, or being a number category that indicates two persons or things, as in Greek, Sanskrit, and Old English.
【語法】 雙數(shù)的：指示兩個人或物的數(shù)字范疇的，如希臘語，梵語與古英語中雙數(shù)概念
n.Grammar （名詞）【語法】
The dual number.
雙數(shù)
An inflected form of a noun, adjective, pronoun, or verb used with two items or people.
雙數(shù)格：用于兩個物體或人時名詞、形容詞、代詞或動詞的曲折變化詞形

來源：
Latin du3lis
拉丁語 du3lis
from duo [two] * see dwo-
源自 duo [二] *參見 dwo-

【引伸】
du.ally
adv.（副詞）