Sites can have multiple pages with the same permalink

[roblillack]

Currently, one is able to have two specific pages with the same permalink, just by creating two page directories which share the same slug, for example: 1.testpage and testpage. Upon building the site, the resulting output directory is written two times without error. This can lead to unforeseen bugs when user's are expecting the content of the “other” page to be generated.

Suggestion solution is to bail out with an error in this situation.