I encountered this error while trying out a writer send-to-publication/ admin approval workflow. I had written about it here:

http://our.umbraco.org/forum/using/ui-questions/1902-Configuring-email-notifications-problem.

But thought best that I'd mention the error I got on the backend when I tried to add a notification for the admin. Here's the message and stack trace.

Multiple controls with the same ID 'r' were found. FindControl requires that controls have unique IDs.

[HttpException (0x80004005): Multiple controls with the same ID 'r' were found. FindControl requires that controls have unique IDs.]
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +273
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.EnsureNamedControlsTable() +61
   System.Web.UI.Control.FindControl(String id, Int32 pathOffset) +222
   System.Web.UI.Control.FindControl(String id, Int32 pathOffset) +327
   System.Web.UI.Control.FindControl(String id, Int32 pathOffset) +327
   System.Web.UI.Page.FindControl(String id) +38
   System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) +232
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1743

I'd also like to mention that I was experiencing some strange error while deleting multiple nodes consecutively very recently. Some of the nodes threw an error while i deleted them. My guess is that it has somehow to with the content tree's refresh functionality.Hence, after deleting a node( say with index i) and then while trying to delete the i+1 node, umbraco confuses the i+1 node to have the id of the ith node. I should probably post this as a bug under a different topic, but thought they might be related.

Hopefully one of you guys know what to do.