It’s not discouraged, either. Having Webmin and Virtualmin is better than not having them, even if we can’t start from scratch on the server, so sometimes we have to deal with weirdness along the way. 
For the sites that show up twice, is one of them SSL and the other regular port 80, perhaps? (That would actually be normal.)
btw i can now go to http://www.hollandsenieuwe.com/about/
and http://www.hollandsenieuwe.com/contact and page source reveals:
<!-- Page not cached by WP Super Cache. No closing HTML tag. Check your theme. -->
did you re-upload the files in binary?
Sorry people, have been terribly busy for clients. I’ll come back on this tomorrow, should have a bit of time then.
About the “http://www.hollandsenieuwe.com/contact” etc: yes, i did reupload in binary, but that wasn’t causing that. What happened was that in the ‘page’ template (not used for everything in wordpress) i was still calling a plugin I’d disabled. That brings me to a silly thing, but maybe a clue: when there’s a mistake in the page (as calling a missing plugin) and not the whole page gets sent to the browser, then that part is shown. I’ve deleted everything in that same page after the missing plugin that could cause an error somehow (I just left tags like </div> where needed etc but it wouldn’t show. So, when there are no errors on the page: blank (in FF and Chrome and some versions of IE). When there’s an error as you spotted (missing plugin near the end): up to that page gets shown.
I’ll ‘speak’ to you tomorrow or maybe tonight. Thanks for all the help!