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I don't think we should be merging the changes
automatically. Person A may not have changed the fields
which person B has changed, but that doesn't person B
shouldn't take person A's changes into account. For example,
person B closing the issue because of lack of response /
information from the submitter, while person A just posted
more information.

I think we should just tell person B that someone has
committed some changes between his reading of the issue and
his changes,  and let him/her decide what to do with those
changes.