Preview stay when clicking on "continue reading"

Hi guys. I just ran into some wierd problem here. I’m running two installations of wp (3.3.1 and 3.0.1 [both only for internal use]) using the theme “Platform” (1.4.2 and 1.3.1). As you see completely different installations. On the frontpage I see a preview of the entries and a link “Continue reading..”. By clicking on the Link I get the entry leaded by the preview. So the preview appears on the actual blogentry.
It kinda looks like this:
<Headline>
<preview>
<complete entry>

Does anybody know about this issue?

regards,
bmn