Line a slow cooker with a liner, if desired.
In a large bowl whisk together orange marmalade, orange zest and juice, soy sauce, ginger, brown sugar, and vinegar.
Add duck to the slow cooker. Pour over a third of the sauce.
Cook on low for 6-8 hours.
Remove duck from slow cooker. Shred with 2 forks and set aside.
Add reserved sauce to a saucepan and bring to a boil. Reduce to a simmer. Whisk together the cornstarch and water. Pour into the sauce pan and whisk together. Simmer until it thickens slightly. Add waterchestnuts and shredded duck to the saucepan and mix together.
Serve over rice and sprinkle with scallions.