Double, Dual
noun, の adjective
When you have two heavy things, you have double and dual the heavy (compared to one, at least). This word is a prefix attached to other words to show that it is "double." 二重 kanji time!
にじゅう
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
分からない文字の下に、二重のラインを引いて下さい。
Please draw a double line under characters you're not sure about.
この魚はニオイが強いので、ラップを二重にしておきますね。
This fish has a strong smell, so I'll double wrap it.
これは、二重アゴをなくすためのエクササイズです。
This is an exercise to get rid of double chins.