With the Email marketing functionality in Online Express, duplicate email addresses are expected to be removed from the email list prior to the email leaving the Online Express email servers. Some users may notice that when they send an email through Online Express, some email recipients receive duplicate copies of the email.
This has been resolved with the May 7, 2019 Online Express release.
Steps to Duplicate
In The Raiser's Edge, navigate to Online Express
Go to the Email Marketing tab and create a new email
On the Get ready to send tab, select your desired email list
Click the Send email button
Follow the prompts
Notice that some recipients received multiple instances of the email