Multiple controls with the same ID ‘xxxx’ were found.

A client rang up last week with a problem with their SharePoint environment.  On their publishing site, every time they went to view the Pages library within Manage Content & Structure they got an error.  Within the ULS logs, this was found:

Unexpected     System.Web.HttpException: Multiple controls with the same ID ‘xxxx’ were found. FindControl requires that controls have unique IDs.

To fix this, I went to SharePoint designer and edited the client’s site.  On the Properties of the Pages library, I noticed there were two views with the same name of xxxx!

I simply renamed one of the views, and I could then see the pages under Manage Content & Structure.  I’m not sure how the client did it, and why SharePoint didn’t prevent this from happening.