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Great question. I'm hoping someone can correct/add to my answer because I'm still trying to understand this too. As far as I understand duality is the state of not understanding that we are one with God. It is seeing ourselves separate than God. It's not seeing everything as part of God. It's an attachment to maya. Maya includes our thoughts so its also when we act via our own ego rather than realizing God's will. When we finally are able to let go of our duality, we are liberated/jivan mukht. The way to getting rid of this duality is stopping attachment to maya. We can stop it by doing simran and sewa, reading our prayers, etc.

Sources: http://spiritualism-asikhperspective.blogspot.ca/2010/08/creator-abstract-theological-god-is.html
http://www.sikhphilosophy.net/threads/what-is-duality.39402/
http://sikhsangat.com/index.php?/topic/44134-doojaiduality/ https://www.sikhnet.com/discussion/viewtopic.php?f=2&t=1793