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1. Open a duplicate (which is actually decided by the program the file is being opened by, and really doesn't do any harm)
Which leads to problem if making changes in one instance, then the other. One lost change is the result.
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2. Simply focus the already-opened file in whatever program it's opened in (again, no harm done)
Not possible. The started program may merely relay a message to yet another program and than just quit. Thus, with some programs, the file would be opened multiple times, with others it won't. I don't like inconsistency.