Duplicity has largely negative connotations as the word is associated with being deceitful or "double-dealing" this could mean making a deal and then backing out at a later date or saying that you will do one thing and then later do something different.
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The Trojan War, in Greek tradition, started as a way for Zeus to reduce the ever-increasing population of humanity and, more practically, as an expedition to reclaim Helen, wife of Menelaus, King of Sparta and brother of Agamemnon.
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Hope it helps :p
Lol, what poem? make sure you post the link