C# - Reload A Dropdown On Postback?

I am populating a dropdown on page load and the user selects an item, click a button (to create a new site in sharepoint) I then want this new site to be shown in the dropdown but I can't get it to work properly. Either the items are duplicated or the selected value is null.

so on page_load I have

getSites();

if the page is a postback I want to reload the dropdown with the new item

if(Page.IsPostBack)
{
getSites();
}

but this of course duplicates all the values so I tried

if(Page.IsPostBack)
{
ddlSites.Items.Clear();
getSites();
}

But even if the site is reloaded with all the items and I select one the value is null, why is that and what should I do to fix this problem?