NOTE:
This option is designed to work only works for manual renewals and NOT for automatic renewals.
Merge or Create new:
On the Payment Form there is a field called "Duplicate Subscription/Members" that has two values, Merge or Create New. Merge will merge any payments to an existing membership if there is enough matching criteria (name, email, payment form, etc). Create New will create a new membership, regardless of if the contact already has existing memberships.
If no value is selected, it will default to using the Merge behaviour.
Disclaimer:
Please note that since this feature does not cater for recurring payments, it will not create a separate recurring payment but instead will connect a new subscription to the existing recurring payment and unlink the old subscription from the recurring payment.