Since WebRatio bases itself on page names and units names, the pages and the units names used in the report pages, must be different. For this reason, in case of identical names, the error that you have indicated, is shown.
You can try to remove from the "WebContent/WEB-INF/descr" folder, the previous report created for this page and synchronize again to create the new report with the new attributes. (I suggest you to copy and save the previous report as backup)
Also you can refer to the article presents on our lean at this link http://www.webratio.com/learn/learningobject/modeling-reports.