Handle duplicate delivery reports to avoid redundant processing

[eskebab]

Description

#1103 identified that duplicate delivery reports can occur. By logging request data to Application Insights, we identified duplicate requests made to the Notifications Email DeliveryReportController. There could be more than one source to the problem of duplicate delivery reports, but by ignoring duplicates that occur at the webhook stage, one could reduce unnecessary processing.

Additional Information

Implementation details will follow. Requires clarification