Duplicate Slugs: Do any WPMUDev plugins require permalink /%postname%/ or is /%category%/%postname

not sure if it is normal behavior but I've got an issue with WP not checking slug duplication between pages and posts. it allows the same slug.

with /%category%/%postname%/ at least I can make links to the post I want.

I know this is going to mess up loading times as WP figures out what postID to return, but searching wp.org forums is fruitless.

1) is it normal for WP NOT to check between pages and posts?

2) is there a way to force non-duplication? (add the -2)?

3) do any WPMUDev plugins require /%postname%/ only ?
(I've got other plugins that do require but might have to drop them)