Salesforce Error: List has more than 1 row for assignment to SObject

Scenario: A subsequent recurring donation Lead did not convert while others previously have been successful, and you see the following error message in the Order detail view of Soapbox Engage:

sbxd.sbx_DonationsRecurringPaymentLead: execution of AfterInsert caused by: System.QueryException: List has more than 1 row for assignment to SObject (sbxd)

It is likely that one or more of the Opportunities for the recurring donation record have multiple contact roles in Salesforce. Each opportunity should have only one contact role.

To resolve the issue:

1. In Salesforce, search the subscription ID found in Soapbox Engage
2. Review each Opportunity record associated with that subscription ID
3. If multiple contact roles exist on the Opportunity record, remove the one that does not have the type "Donor"

Once the above steps are complete, go to the order detail view in Soapbox Engage and click "Convert Lead." The automatic lead conversion process should be successful.