URLs with a trailing slash are seen as separate page from their non-slash equivalents. Trailing slashes are usually the result of a typo or someone manually editing the URL (for example, to get to a parent page).
These should be the same resource:
Apparently, the proper fix is to do this in Apache:
RedirectMatch permanent ^/wiki/(.*)/$ /wiki/$1
*** This bug has been marked as a duplicate of bug 20290 ***