When opening the following link with Claws Mail, it opens a window to write a new message, but the CC and BCC recipients are missing. <a href="mailto:Mr.%20Test%20%3CMr.Test@something.com%3E,Mrs.Test@something.com,Junior%20Test%20%3Cjunior.test@something.com%3E?cc=Mrs.Test@something.com,Junior%20Test%20%3Cjunior.test@something.com%3E&bcc=Mrs.Test@something.com,Junior%20Test%20%3Cjunior.test@something.com%3E&subject=This%20is%20an%20example&body=Hi%2C%0A%0Athis%20is%20a%20test."> test</a> Maybe Claws Mail removes the Mail addresses as they are duplicates, but this is not the behavior that users would expect.
Is that not what users would expect? Under what circumstances would sending the same message several times to the same address make sense? Claws Mail will add Cc and Bcc address from such a link if they are not already included.
I think there are edge cases in which it makes sense and the point here is, that the link explicitly requests adding the duplicates. (I also compared the behavior with some other mail clients, Claws Mail was the only one that filtered the addresses.)
Maybe it would be a better solution to highlight duplicates or to ask the user if he wants to remove them.
More likely than not, the server will only deliver one copy to its recipient anyway, so the "edge cases" are theoretical and asking the user is a waste of everybody's time.