<asp:RePEater ID="Repeater1" runat="server">
<ITemTemplate>
<%# ((KeyValuePair<int, List<User>>)Container.DataItem).Key %> <br />
<asp:Repeater ID="Repeater2" runat="server" DataSource='<%# ((KeyValuePair<int, List<User>>)Container.DataItem).Value %>'>
<ItemTemplate>
<%# (Container.DataItem as User).Id %>
<%# (Container.DataItem as User).Name %>
</ItemTemplate>
</asp:Repeater>
<hr />
</ItemTemplate>
</asp:Repeater>

public partial class Temp : System.Web.UI.Page
{
Dictionary<int, List<User>> dict = new Dictionary<int, List<User>>();
PRotected void Page_Load(object sender, Eventargs e)
{
dict.Add(1, new List<User>
{
new User{Id = 1, Name = "aa"}
,new User{Id = 2, Name = "bb"}
});
dict.Add(2, new List<User>
{
new User{Id = 3, Name = "cc"}
,new User{Id = 4, Name = "dd"}
});
Repeater1.DataSource = dict;
Repeater1.DataBind();
}
}
public class User
{
public int Id{get;set;}
public string Name{get;set;}
}