I posted my reply in the support ticket you have opened on the same topic. For convenience I am pasting it below:
The problem occurs because the upload is initialized twice when used in the Window content. This is already fixed in the latest internal builds that are available for you to download from your account. Please update the version you are using and let me know if the problem persists.
the Telerik team
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI