HOW TO FILL COMBO BOX IN VB.NET - vb.net

Question

I have a code to fill my combo box but every time i close the form the list is being doubled, if i have a list from my database of English, Mathematics, Science after i close the form and open it again the list showing is now English, Mathematics, Science, English, Mathematics, Science
HERE IS THE CODE,
Call OpenDB()
cmd.CommandText = "select * from Subject"
cmd.Connection = conn
dr = cmd.ExecuteReader
While dr.Read()
cmbSubject.Items.Add(dr("Subject Name"))
End While
dr.Close()
Call CloseDB()

Answer 1

The problem is not in the method you are using to bind the combobox. in each time it binds that's why you are getting duplicate records in the database. to avoid this please clear the combobox before each bind, like the following:
Call OpenDB()
cmbSubject.Items.Clear ' extra statement added to clear the item collection
cmd.CommandText = "select * from Subject"
cmd.Connection = conn
dr = cmd.ExecuteReader
While dr.Read()
cmbSubject.Items.Add(dr("Subject_Name"))
End While
dr.Close()
Call CloseDB()
If you need an alternate method for binding the combobox i will suggest you binding with Dataset
following is the example code for this :
Dim SQL As String= "select Subject_Name,Subject_code from Subject"
Dim adapter As New OdbcDataAdapter(SQL, conn) '<-- This function will Retrieve Data and Return as Dataset together with table name
Dim myData As New DataSet
myData.Fill(lobjDataSet, tblName)
cmbSubject.DataSource = ds_myData
cmbSubject.DataMember = "Subject"
cmbSubject.DataTextField = "Subject_Name"
cmbSubject.DataValueField = "Subject_code"
cmbSubject.DataBind()
myData .Dispose()

Answer 2

Public Class frmRegistration
Dim obj As New Entity.Registration
Dim bus As New Business.Registration
Private Sub BtnSave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnSave.Click
BindingSource1.EndEdit()
bus.Save(obj)
RefreshGrid()
MsgBox("Saved.")
End Sub
Sub RefreshGrid()
Dim dt1 As DataTable = bus.Select()
BindingSource2.DataSource = dt1
DataGridView1.AllowUserToAddRows = False
DataGridView1.DataSource = BindingSource2
End Sub
Private Sub Registration_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
RefreshGrid()
NewItem()
End Sub
Private Sub BtnNew_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnNew.Click
NewItem()
End Sub
Sub ResetDis()
BindingSource1.DataSource = obj
End Sub
Sub NewItem()
obj = New Entity.Registration
ResetDis()
End Sub
Private Sub BindingSource2_CurrentChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BindingSource2.CurrentChanged
obj = New Entity.Registration(CType(Me.BindingSource2.Current, DataRowView).Row)
ResetDis()
End Sub
End Class