I know Craft appends slugs with -1 if they already exist but I'm working on an app where the addition makes things awkward for us (we have to match the slug to a third-party service). I was under the impression they had to be unique but it seems you can edit an entry and change the slug to be the same as one that already exists.
This could be a good workaround for us, but I want to check whether it's a bug that's likely to get squashed or deliberate behaviour.