#577 multiple instance of document opened

[Anthony Knittel]

after i've been opening and changing between a number
of files for a while i have realised there are two
instances of a particular file open. ie two tabs in
the editor pane representing the same file.

each instance has a separate working data set, which
means if you make some changes to one and some to the
other, data will be lost. this makes the editor
completely unusable for me, as i cannot trust that
changes i make during normal editing will not be lost.

(running 4.9.9.2)