UPDATE: As of May 20, 2021 transaction participants no longer require unique email addresses.
Remine Docs identifies each recipient by their associated email address, so each party member (the buyer(s)/seller(s), the cooperating agent, etc.) needs their own, unique email address in order for you to send them any documents.
As an agent, you may find yourself working with, say, a married couple who share an email address: you've got two people who need to sign, but only one email address between them. The most obvious solution (having the recipient create a new, free email account for the purpose of the transaction) is probably the easiest, for a number of reasons. An alternative, depending on the email domain (eg. @gmail.com), you can append the email with a plus ("+") sign and any word or number before the @domain end of the email address. So, for example, if my client's email address is firstname.lastname@example.org, and I used email@example.com for the client's spouse, they'd still receive the email and Remine Docs sees it as unique. This doesn't necessarily work for every email domain, though, and you'll want to make sure that it works before sharing documents with the "workaround" email. More on this workaround (pertaining to gmail accounts) here.
"But the email addresses are different"
When an issue like this is experienced, it's typically because people and roles across all documents in the transaction folder are not consistent. You'll want to start by going back to the transaction folder, opening a document and click 'People' at the top. Once there, make sure that you've got everyone in the right role, including yourself. Then, click the next document in the left side margin, and again, check that document's 'People' panel. Repeat until you've double-checked all documents. The issue should resolve after checking all documents and their people.