How to solve many to many relations tables entity framework

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entity-framework

Relation tables insert but I take a error.How to solve this problem?

This error is

an entity object cannot be referenced by multiple instances of ientitychangetracker.

What I have tried:

var 
studentClassNumber=DContext.Instance().StudentEntities.tblStudentInformation.ToList();
DContext.Instance().StudEntities.tblStudentInformation.AddRange(studentClassNumber);
DContext.Instance().StudEntities.SaveChanges();

This error is take bold row
an entity object cannot be referenced by multiple instances of ientitychangetracker.