Reply-all includes the original recipient
When using the Reply-All feature, Evolution includes the original recipient (now the sender) in the To: field. This results in the sender receiving a copy of his/her own email. The user then needs to either manually remove his/her email address from the To-field, delete the unnecessary email, or just leave the redundant email in the folder.
The solution is to remove the sender's email from the To-field.
This problem is evident in Evolution 3.28.5-0 on Ubuntu 18.04.1