How to solve exception in C#.net

I have combobox referring the "Book_id" field of Book table from bookbindingsource dataset. when I select option, I'm shown this message "Column 'Book_id' is constrained to be unique. Value '899866238' is already present." and in the troubleshooting exceptions portion, there is written "Relax or turn off constraints in your DataSets. Make sure you are not trying to assign a value to a primary key field where the primary key already exists in the data table". This is not clear to me. Can anybody help me? Regards, Shomen