Why do I see two transports in the log when I have spamassasin enabled ?

When spamassasin is enabled, the message is reinjected back into the mail queue so it can be passed though the system filters/user filters. You will only get one delivery to your inbox, but exim will process the message twice.

[msg]----[exim]-----[spamassasin]-----[exim]-----[delivery]