your question "smells" for "using server-side includes". In these threads David_Powers gave me a wonderful hint:
And after that (I quote David_Powers from http://forums.adobe.com/thread/561892): Several people have asked about this in the past, so I decided to create a quick tutorial. It's at http://foundationphp.com/tutorials/sprymenu/ssi.php.
In my understanding, whenever you edit the contents of a library item, it will update all other occurances of this library item on the rest of the site. That's "working as designed" as far as I know, unless you break the link by telling it to not update - which kinda defeats the purpose I think.
But theres another way to do this using template properties, i#ve been using this approach myself a couple of times:
Write your navigation bar in the template (.dwt) file itself, then select the class= of your first menu item and choose .. (names might be off, I am using a non-English copy of DW): "modify > template > make attribute editable" from the main menu. Do so for all your menu items and name these new template properties something like "item_x_class"
You'll end up with something like this in your template source:
and a matching
<!-- TemplateParam name="item_1_class" type="text" value="active" --> in the header
Now, on each page, you can set the class within the template properties dialog ("modify > template properties"). If your main menu changes, it won't overwrite your template properties, but still update all pages when you do make a change to the menu.
If you're using multiple templates for your site, consider nesting templates, so the master template already includes your navbar and all subsequent templates will "inherit" changes to the navbar code (sans the editable class property).
Hope this helps.. or at least gives you an idea for further experiments..