It is usual for a fluid template to get rendered completely when not using a layout and sections. In your adaption of modernpackage, make sure that on top of your main template the line
<f:layout name="Default" />
is included which will use the file "Default.html" in the Layouts subfolder of the extensions Template folder as a layout. The file will contain only one line:
<f:render section="body" />
By doing this the only thing in your main template that will get rendered is anything in between these tags:
<f:section name="body">...</f:section>
Anything before or after will now be ignored and the double rendering will be prevented.