how to prevent from Duplicate data entry?

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles addbtn.Click

    If myconn.State = ConnectionState.Open Then
        myconn.Close()
    End If
    
    myconn.Open()
    Dim i As Integer = mycom.ExecuteNonQuery()
    Dim Q As String
    ''-------------------------------------------------------------------------------------
    ''-------------------------------------------------------------------------------------
    
    Q = "INSERT INTO tblelogin(Username,name,Password,Accounttype) VALUES('"
    Q = Q + txtuname.Text + "','" + txtname.Text + "','" + txtpass.Text + "','" + accountbox.Text + "')"
    
    If txtpass.Text <> txtconfirm.Text Then
        MsgBox("password mismatch. please enter again.")
        txtpass.Text = ""
        txtconfirm.Text = ""
    
    End If
    
    If txtuname.Text = "" Or _
        txtpass.Text = "" Or _
        txtname.Text = "" Or _
        txtconfirm.Text = "" Or _
        accountbox.Text = "" Then
        MsgBox("input data on the field.")
        
        Exit Sub
    Else
    
        mycom.CommandText = Q
        
        i = mycom.ExecuteNonQuery()
        If (i > 0) Then
            MessageBox.Show("Record is Successfully Inserted")
        Else
            MessageBox.Show("Record is not Inserted")
            
            myconn.Close()
        
        End If
    End If