Slug doubles page name

Link please? That may make it easier for others to figure out what’s going on there.

In the meanwhile, if you temporarily switch your theme to the Twenty Twelve theme does this issue still exist for you?

If I understand you (and I’m not sure that I do) then it looks like you’ve two things going on. The links on top of your pages are your navigation menu.

http://codex.wordpress.org/WordPress_Menu_User_Guide

The pages however do have the title being displayed also.

http://www.hetgewildewesten.nl/New/over-ons

You can try to hide that using CSS.

.entry-title {
    display: none;
}

That can be added via custom CSS plugin (I’ve been using the one via Jetpack lately) or by creating a child theme of your theme and adding that CSS there.

Don’t edit your theme directly as you’ll lose your changes when the theme gets updated.

Glad to help! Just make sure you add that CSS using one of the methods I described. Wouldn’t want you to lose your changes. 😉