If you create an entry with a name that is already used within the weblog, EE will append a # to the URL title which makes this number unique. E.G. If there is already a story called “Election Day”, the second entry with the same name will receive a URL title of “election_day1”
This check isn’t however performed across weblogs. So if an entry in weblog 2 tries to use the same title as an entry in weblog 1, nothing special will happen, and permalinks to both stories will now crap out.
