Duplicate Contacts created by Webform

This fault was first found on version 7.x-4.28, but is still present in 7.x-5.0.
I am using Drupal 7.69 and Webform 7.x-4.21

The circumstances to create the problem are a Webform set up to input:

• An Individual Contact (Contact 1) linked to a Household Contact (Contact 2) by a ‘Household of/Member of’ Relationship.
• The Household has a paid-for Membership. In practice this propagates down to the Individual, but the same problem occurs if it is set up without propagation.
• The Individual Contact 1 has two email address fields.
• A new pair of Contacts 1 and 2 are created using this form with the second Email field (ONLY) filled in.
• Payment is made using either the Dummy Card Payment Processor, the Stripe PP or a custom Processor we have implemented to record cheque payments (essentially by sub-classing the core basic PP).

The result is that:

• The Household and its Membership are created correctly.
• An Individual Contact is created against which the Contribution is recorded. This does not have a Relationship set up, and hence no Membership.
• A Second Individual is created which has the correct Relationship to the Household, and hence a Membership (if propagation is configured), but no Contribution.

From the Contact Id numbers the creation sequence is: Individual with Contribution; Individual with Relationship; Household.

If only the first email address, or both, are filled in then the system behaves correctly.