It’s there to let people know that you went into offline mode, instead of just logging off, so that they are alerted that you’re hiding from them. Pretty sure it’s been like this since they added the offline mode. Realistically, the double spam is due to you going offline for both WoW and Bnet at the same time, so it shows it twice to people. It’s likely too much hassle to implement a simple do-once with a small cooldown timer for events like this where some notification can happen multiple times within a second.