I was editing a question titled "newton's law of restitution" and I edited the title to "Newton's law of restitution" (There were latex issues in the body of the question and I was not editing the question just because of obsessing over the title as it might seem from my recent behavior :P). When I finished the edits and tried to post them, the system gave me an error that a question with an identical title already exists. The only difference between the old and the new title was capitalization of "N" in "Newton". I think this is a serious issue if someone can smuggle a question with an existing title via tweaking trivial capitalizations. Also, in such a situation, should one simply notify the OP that while the system is not registering an error, they should change the title or should one change the title themselves so as to suit the question? I did the latter--I am not particularly sure I should've done it. This might be a network-wide issue--if so, can this question be migrated or should I ask on the central meta separately?
If two questions are not duplicates of each other and yet their titles only differ in capitalization, then the title is bad and should be changed (preferably on both questions) to a title that is more specific to the individual questions. See How do we write good question titles? for, well, how to write good question titles.
Everyone is free to edit other people's posts on Stack Exchange. While you should be trying to preserve authorial intent, if you genuinely believe you've got a better title, just go ahead and edit it in. Do not hesitate to improve other people's posts! (but be sure that it's actually an improvement before doing so)
If the questions are duplicates of each other, then it's unavoidable that their titles might be similar. Don't sweat improving the title of a question that's closed as duplicate since e.g. users that are not logged in are automatically redirected to a duplicate's target anyway.