After every postback dropdownlist items repeats - asp.net

Question

I have bind my dropdownlist with database. But on each PostBack the items in dropdownlist gets repeat again.
e.g.
OnPage Load I have this items in dropdownlist 1,2,3,4.. etc Now suppose page gets postBack then it looks like 1,2,3,4,1,2,3,4
Vb code
Private Sub hospitals_Load(sender As Object, e As EventArgs) Handles Me.Load
Me.PopulateAreas()
End Sub
Private Sub PopulateAreas()
If IsPostBack Then
Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
Using conn As New MySqlConnection()
conn.ConnectionString = ConfigurationManager _
.ConnectionStrings("conio").ConnectionString()
Using cmd As New MySqlCommand()
cmd.CommandText = "Select * from areas where areaCity Like '" + citySelector.SelectedItem.ToString + "%'"
cmd.Connection = conn
conn.Open()
Using sdr As MySqlDataReader = cmd.ExecuteReader()
While sdr.Read()
Dim item As New ListItem()
item.Text = sdr("areaName").ToString()
item.Value = sdr("areaID").ToString()
'item.Selected = Convert.ToBoolean(sdr("IsSelected"))
areasList.Items.Add(item)
End While
End Using
conn.Close()
End Using
End Using
End If
UPDATE
I have master page which has dropdownlist of cities. I am using masterpage control to mycontent page like below.
Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
Now in my PopulateArea() class there is query in which WHERE clause have cityselector. So according to city my area gets fetched.

Answer 1

Call your function inside not postback event so that the dropdown is not called on postback events(which fills it everything).
Private Sub hospitals_Load(sender As Object, e As EventArgs) Handles Me.Load
if ispostback = false then
Me.PopulateAreas()
end if
End Sub

Answer 2

its pretty simple.. the page load event is triggered every time anything is post back.
use this
Private Sub hospitals_Load(sender As Object, e As EventArgs) Handles Me.Load
If Page.IsPostBack Then
//do stuff when post back occurs
Else
Me.PopulateAreas() //put this function here so that it executes only once
End If
End Sub

Answer 3

If your dropdown values have to change on the postback. Say initially the values where 1,2,3,4 and on postback if the values has to be 2,3,4,5 based on some of the values you change on the form controls you will have to clear the dropdown values first and then add new values to it.
areasList.Items.Clear()
Also see that AppendDataBoundItems is not true: in your .aspx page

Answer 4

Change your code to,
Private Sub PopulateAreas()
If IsPostBack Then
areasList.Items.clear()
Dim citySelector As DropDownList = Page.Master.FindControl("locationSelector")
Using conn As New MySqlConnection()
conn.ConnectionString = ConfigurationManager _
.ConnectionStrings("conio").ConnectionString()
Using cmd As New MySqlCommand()
cmd.CommandText = "Select * from areas where areaCity Like '" + citySelector.SelectedItem.ToString + "%'"
cmd.Connection = conn
conn.Open()
Using sdr As MySqlDataReader = cmd.ExecuteReader()
While sdr.Read()
Dim item As New ListItem()
item.Text = sdr("areaName").ToString()
item.Value = sdr("areaID").ToString()
'item.Selected = Convert.ToBoolean(sdr("IsSelected"))
areasList.Items.Add(item)
End While
End Using
conn.Close()
End Using
End Using
End If
End Sub