A duplicate value cannot be inserted into a unique index error
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**I have a sql server compact table with an ID column which types is "uniqueidentifier" and "Is RowGuid". This column is primary key also. I connect this table with entity framework. I create and add a new row with following statements:**

    Guid l_guid = Guid.NewGuid();
    TableName cp = new TableName ();
    cp.ID = l_guid;
    cp.ColA= ColAValue;
    cp.ColB= ColBValue;
    cp.ColC= ColCValue;
    Entities.TableName.AddObject(cp);
    Entities.SaveChanges();

**The program that this code run in have several hundred objects and each object has these statements in an instance method. When a specific property of these objects changes (has a desired value), that object saves its properties (ColA, ColB, ColC) to the database with the satements that I wrote above.
Some times at SaveChanges() it gives following error:**

    A duplicate value cannot be inserted into a unique index. [ Table name = TableName,Constraint name = PK_TableName ]

**Why does this situation occure? What do I do wrong?**