When I'm trying to send an email where multiple attachments with the same name, the receiver thinks that it is always the same attachment and shows the same content for all attached files.
I have tested this with versions 6.5.2 and 8.3.1.
In my opinion, the problem is the assignment of the BodyPart ID in org.simplejavamail.converter.internal.mimemessage.MimeMessageHelper.getBodyPartFromDatasource(AttachmentResource, String)
(line 288) where the Content-ID header of the part is based solely on the filename of the attachment. I suggest to always add a UUID at the end to ensure uniqueness.