Depuplication problem with multiple Contacts

Hello.

I have a webform containing (in order): an Individual Contact; a Household Contact with a Relationship to the first Contact; and three optional 'Additional' Individuals with Relationships to the Household. There is also provision for selecting a Membership.
We are migrating an existing offline database to CiviCRM, and most of the membership administration is still being done by an admin team offline - not (yet) by members going online themselves.

If an administrator, in error, completes a blank webform with details of a person and the household who are already recorded in the system, i.e. enters them as a new member, rather than updating the existing record, then:
-- The Household Deduplicates correctly.
-- The Individual Contact does not, and a second Individual is created, related to the one Household alongside the existing Individual.
-- If the Dedup rule for Individuals is then run manually it finds the match correctly.

If, as a test, one loads a webform with an existing Household and related Individuals, then adds a duplicate person as another Additional Individual, it deduplicates correctly.

The Dedup Rule matches on First Name, Last Name and email address.

With thanks.