You can copy existing sent, draft, or scheduled email from the Email & Text page.
If you have access to multiple departments, you may need to click the department first.
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Locate the email you would like to copy, and click the 3 dots (⋯) that come after the email's subject line:

Select Copy to a new draft from the drop down menu that appears.

This will open a new copy of the email, where you should automatically be assigned as an admin. You can edit or add additional campaign admins, recipients, and edit the email content to suit your needs.
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Note: The email will be copied in the same department that it originated from.