How can I avoid duplicates insert of records from datagridview to SQL Server table - sql-server-2008-r2

Question

I have a table in SQL Server 2008 R2 where I have a table CT11. What I want is to stop inserting duplicates records in the table using cellendedit event. How can I do it because I am getting duplicates of records?
Private Sub DataGridView1_CellEndEdit(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellEndEdit
For Each row In DataGridView1.Rows
If cmbexam.Text = "CAT 1" Then
sqlSTR = "INSERT INTO CT11 (Admno, Name, Score) VALUES ('" & row.Cells(0).Value & "','" & row.Cells(1).Value & "','" & row.Cells(2).Value & "')"
ExecuteSQLQuery(sqlSTR)
End If
Next
End Sub

Answer 1

First, learn how to use SQL Command
How to use parameters "#" in an SQL command in VB
If you want the database to validate your entry then learn how to use Stored Procedure
https://mostafaelmasry.com/2015/10/03/creating-system-stored-procedure-in-sql-server-2008-r2/
In Stored Procedure add a condition that will check your entry ex;
SELECT #record = COUNT(1) FROM CT11
WHERE Admno = #Admno AND Name = #Name AND Score = #Score;
IF #record = 0
BEGIN
INSERT INTO CT11 (Admno, Name, Score) VALUES (#Admno, #Name, #Score);
END
In VB
For Each row In DataGridView1.Rows
If cmbexam.Text = "CAT 1" Then
MyCommand = New SqlCommand("StoredProcedureName", dbConn)
MyCommand.CommandType = CommandType.StoredProcedure
MyCommand.Parameters.AddWithValue("#Admno", row.Cells(0).Value)
MyCommand.Parameters.AddWithValue("#Name", row.Cells(1).Value)
MyCommand.Parameters.AddWithValue("#Score", row.Cells(2).Value)
MyCommand.ExecuteNonQuery()
End If
Next
Or follow this thread: SQL Server stored procedure and execute in VB.NET
Or you can use VB to validate the values by executing a select statement and check if there a record or not.