AJAX Accordion Control - Multiple controls with the same ID '_header' were found. FindControl requires that controls have unique IDs.

This is one of the common error we get when you use multiple panes in Accordion control. 

The problem is with duplicate ID for the panes. Setting unique ID's to dynamic panes will fix this issue

<cc1:accordion id="Accordion1" runat="Server" selectedindex="0" 
    autosize="None" fadetransitions="true" transitionduration="350"
  framespersecond="50" requireopenedpane="false" suppressheaderpostbacks="true">
</cc1:accordion>
In Code behind based on the number of panes:

for (int i = 0; i < 2; i++)
{
    AccordionPane pane1 = new AccordionPane();
    //use Guid here for uniqueness
    pane1.ID = "AccordionPane" + Guid.NewGuid().ToString();
    Label Label1 = new Label();
    Label1.Text = "New pane";
    pane1.Controls.Add(Label1);
    Accordion1.Panes.Add(pane1);
}