Multiple controls with the same ID '_header' were found. FindControl requires that controls have unique IDs.
I got this error, when I tried adding multiple accordion panes to the accordion control in code behind. The problem is with duplicate ID for the panes. Setting unique ID's to dynamic panes will fix the problem.
<cc1:accordion id="Accordion1" runat="Server" selectedindex="0"
headercssclass="accordionHeader" contentcssclass="accordionContent"
autosize="None" fadetransitions="true" transitionduration="250"
framespersecond="40" requireopenedpane="false" suppressheaderpostbacks="true">
</cc1:accordion>
In Code behind:
for (int i = 0; i < 3; i++)
{
AccordionPane pane1 = new AccordionPane();
//Here i used Guid for uniqueness
pane1.ID = "AccordionPane" + Guid.NewGuid().ToString();
Label lbl = new Label();
lbl.Text = "New pane";
pane1.Controls.Add(lbl);
Accordion1.Panes.Add(pane1);
}