How to register startupscript mutiple times

Hi All,
I have .aspx page in that there is a which calls a js funtion which shows a popup. when user clicks on a button in that popup
i wanna display another popup / alert msg from code behind. I am trying

ScriptManager.RegisterStartupScript(this, this.GetType(), Guid.NewGuid().ToString(), "alert('a msg');", true);

and

Page.ClientScript.RegisterClientScriptBlock(divSearchUp.GetType(), "msg", "alert('a msg');", true);

and

if (!Page.ClientScript.IsClientScriptBlockRegistered("msg"))
{
Page.ClientScript.RegisterClientScriptBlock(divSearchUp.GetType(), "msg", "alert('a msg');", true);
}

but non of them are working.