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 jane.smith@gmail.com for a recipient, and you want to add her twice, you could use jane.smith@gmail.com and jane.smith+hr@gmail.com as two separate emails, and they would both end up in the same inbox (the one of jane.smith@gmail.com).

Other examples might include:

• mark-anderson@mycompany.com 
• mark-anderson+2@mycompany.com 
• mark-anderson+3@mycompany.com 

or

• angela.oberin@acme.com
• angela.oberin+sales@acme.com
• angela.oberin+management@acme.com

etc