Why a  duplicate opportunity is created after the customer creates a subscription using checkout link generated from the opportunity ?

Summary:

Sales Rep intends to create a subscription in CB from Salesforce by using the create/change subscription button and sending a checkout link. Upon completing the checkout process, they expect that following the next sync, the original opportunity will transition to "closed won." However, instead of the original opportunity being updated, a new opportunity is generated and automatically marked as "closed won."

Clarification:

The Sales Rep might be  bypassing the email-sending process by directly copying the checkout link from the email draft generated in Salesforce. This activity results in a duplicate opportunity creation.

Email-sending is an integral part to complete  the entire workflow.

Once the email is sent, it will be recorded under Activities and ' CB checkout link' related list.

The subscription created will be associated with the opportunity(rather a duplicate) from which the checkout link was generated.