Include CSS Only If It Isn't Already Included?

I use the code bellow to dynamically include a CSS file:

HtmlHead head = (HtmlHead)Page.Header;
HtmlLink link = new HtmlLink();
link.Attributes.Add("href", Page.ResolveClientUrl("~/App_Themes/Default/StyleSheet.css"));
link.Attributes.Add("type", "text/css");
link.Attributes.Add("rel", "stylesheet");
head.Controls.Add(link);

The problem is: I want to do it only once, and only if it isn't alrealy included in the page.

How do I verify if it is already included?

Edit:

Answers telling me to include in page load using !IsPostBack won't solve my problem, as this code will be inside a Web User Control and my page may have a lot of the same user control.

For example, I use the code below to do it with javascript:

if (!Page.ClientScript.IsClientScriptIncludeRegistered("jsScript"))
{
Page.ClientScript.RegisterClientScriptInclude("jsScript", ResolveUrl("~/Utilities/myScript.js"));
}