NAV 2013 & Later - An item with the same key has already been added.

Dear All,

During a upgrade process to NAV 2013 R2 from NAV 2009 SP1, i faced an issue which took a lot of time to resolve.

So i thought sharing the same with you all.

When i tired to run a page it crash the NAV 2013 Windows Client exe, I thought that page might have issues but when i run table from developer environment everything same.

The ERROR that i was getting - "An item with the same key has already been added"

Below is the Some part of Error in event viewer -

System.ArgumentException was unhandled
  HResult=-2147024809
  Message=An item with the same key has already been added.
  Source=mscorlib

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