DataGridView table

Hi all i use a data-grid view table in the form.Once i enter the data first time ,it's ok,but then i try to modify certain row and then i exit the form,form gets updated and duplicate result on the top of existing data.So each time i try to update row,,after exiting the form,i get duplicates .I use .bin file instead the database, that's the requirements.How to avoid this duplicates being recorded on the top of existing data?Thanks