Duplicate attachments being received

Sometimes when I send Apple mail with attachments the recipient tells me that they get duplicates of just one of the attachments. When I look at sent mail through my Google account the attachments are all different but today I sent the Apple mail to myself as well and indeed, all attachments are the same. Any ideas?

Image 1: screenshot showing Apple Mail received showing same attachment three times

Image 2: screenshot showing Sent mail (viewing in GMail account)