IList lst=new ArrayList();
The above line is why it is replacing your first submit value.
You are recreating the entire list, and then giving it the only value of the new user you submitted.
Make the lst field a class member, then do this:
if( lst == null )
{
lst = new ArrayList();
}
lst.add(user);
GridView1.DataSource=lst;
GridView1.DataBind();