Duplicating some of the controls on the page could be due to the code-behind if controls are added or removed from the page dynamically. From the provided snippet it would be hard to determine what could be causing the problem.
Could you elaborate more on the scenario you have? There is a RadAjaxManagerProxy so I am guessing the code is from UserControl. Is the whole user control duplicated on the page or just the repeater? Does the problem occur with a standard Asp Repeater also?
Try disabling Ajax on the page by commenting out RadAjaxManager or setting its EnableAJAX property to false and see if any error is thrown. I would appreciate it if you could share your full markup with the code-behind. This would help us have better understanding of your scenario and could try to replicate the problem locally.
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