@Shoham as a wild guess, maybe this is about how the attachments are included in the email, i.e. if there is some duplication of the information that the attachment is attached. Could you check the raw message body (right-click the message header) and choose "show raw message", and then look for the lines that are something like
Content-Type: multipart/mixed;
boundary="006_FR0P281M[........]DEUP"
As far as I understand how these things work, for every attachment there is such a Content-Type definition up there in the header and the boundary is used for clients to understand where the attachment data starts & ends.
So my wild guess is that the original email contains the data once but the boundaries appear twice for every attachment, and when the email is forwarded Vivaldi adds the attachments for every noted section. Again, super wild guess.