Merge Recipients and delete orphanded

Here is the case with AC Classic 6.1:

I have several custom fields for nms:recipient, and all of them are controlled by external DB. E.g. AC receives CSV file from external system and creates nms:recipient records from this file.

At the same time, landing page hosted on AC also creates nms:recipient records.

Because of this I have several (usually 2) nms:recipient records with same email, but one of them are not controlled by external DB (doesn't have values in custom fields).

What I want is to somehow tell AC to:

1. Find all nms:recipient records with same email

2. Merge them into ones where record with custom field (e.g. @externalSystem is not empty) is master record and all broadLogs should be linked to that record

3. Delete all orphaned records without @externalSystem field.

I know there is an 'Update and merge collections' action in Update Data activity, but I can't understand how exactly it should be configured. Will appreciate any advice.

Thx