If you want to add multiple recipients with the same email address, you can use a simple trick that works with quite a few mail servers (including the ones from Gmail or Outlook.com):
use "+something" in your email address
For example, if you have the email address email@example.com for a recipient, and you want to add her twice, you could use firstname.lastname@example.org and email@example.com as two separate emails, and they would both end up in the same inbox (the one of firstname.lastname@example.org).
Other examples might include:
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