What can be considered as a “very similar” proposal?

It depends because some proposals can be very similar, for instance in the case of resubmission; and others are not quite similar, but they are still considered similar, for instance in the case of continuation. Even if the applicant mentions that a similar proposal has been submitted, this will be checked very carefully by REA. If after checking REA sees it is not the case, they will not consider it to be similar.