Hello,
Are they using the webmail interface or a desktop mail client with gmail setup for pop (imap?) and smtp for sending out through the gmail servers? Whichever is being used for sending, the gmail webmail or a desktop mail client, please check it for filters and other messaging rules that would automatically be sending a copy with CC or BCC to themselves. Is it possible there is some other software involved such as a desktop virus scanner that could be sending a copy after scanning? I'm not sure if that's a common feature but it's possible. The "keep a copy" of email messages policy happens sometimes within corporate so I'm adding that as an option since you mentioned it only seems to be happening when they are checking their email from their work computer.