Already Open file warning

[JoseOlin]

Sometimes I accidentally open the same file in two instances of remarkable and made modifications only in one of them. But before closing the unmodified one I save again which discard all of the modifications. If a file is opened twice would be good to be detected and show up the already open instance or at least show a message of the kind "this file has been changed outside of this window" or something.

Best regards and keep the good work!