There is a second one of you walking around. Same face, same build, same way of standing — but you are here, so who is that? Every use of the word sits on that one moment of looking at a copy of a person. The only thing that changes is what the copy turns out to be: a spirit, a stranger, or a creature wearing a stolen shape.
可以想成世界上有第二個你正在走動。一樣的臉、一樣的身形、一樣的站姿。可是你人在這裡，那邊那個又是誰？這個字的每一個意思，都停在「看見一個人的複製品」的那一瞬間。變的只是那個複製品最後是什麼：一個靈體、一個陌生人，或一隻披著偷來的形體的怪物。
From the word’s history
Borrowed from German Doppelgänger, literally double-walker: Doppel is double and Gänger is one who walks. The word itself says there is a second one of you out walking, which is exactly the picture every sense keeps.
這個字是從德文 Doppelgänger 借來的，字面是「雙重行走者」：Doppel 是雙重，Gänger 是走路的人。字本身講的就是「有另一個你在外面走動」，而這正是每個意思都保留下來的畫面。
Wiktionary, "doppelganger"
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