PDF and RTF export print twice the same page if given the chance.

Problem:
If you pass to the PDF or RTF export URL the variable pages twice with the same page, no checks are made to uniquely print pages so the resulting document has the same page printed twice on it instead of having it only once.

Note: The XAR and HTML exports work seem to be working fine since they operate with files.

How to reproduce:
http://www.playground.xwiki.org/xwiki/bin/export/Main/WebHome?format=pdf&pages=Main.WebHome&pages=Main.WebHome
http://www.playground.xwiki.org/xwiki/bin/export/Main/WebHome?format=rtf&pages=Main.WebHome&pages=Main.WebHome