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If Organisation B invite the patient with no NHS number, there is no way to match to the patient's record in Organisation A and a second record will be created. See 'How PKB matches to existing patient records' for more details on how PKB matches to existing records. For HL7 our matching rules can be found here
If the patient has an email address in their record, they will be sent an email notification informing them that your team now has access to their record.