On the classic version of Freshdesk, this error means that the email address you're trying to save is already saved as either a contact or agent in Freshdesk. When you get the error, you'll find a link next to it to 'view user' which would redirect to the contact's profile.
Please use the option 'Convert to agent' on the top right side of the page to add this contact as an agent. Once this is done, please navigate to Admin > Agents > click on Edit next to this newly added one > and change the profile details of this particular agent.
If you are using Freshdesk on Mint, once you enter the email address, the system will check if this is an existing user on your account. If so, you will be notified of the same. You can search for the contact and use the 'Convert to agent' button to convert that contact into an agent.
If the email address is new, you will get the option to enter further details - Full name, Work number, Mobile number and Title.