This isn't a big deal, but it's just a little annoying.
When I go to my Gmail inbox in my browser on my computer, my phone plays the email notification. It's not even triggered by opening the Inbox page exactly, because even if I have a tab open with my Gmail Inbox open it will wait until I actually focus that tab and click or do something on the page itself, then my phone will play the notification sound.
What is it about focusing the Gmail inbox page on my PC's browser is causing my phone to play the email notification sound? Keep in mind this is also after it has already played the sound to tell me I have email, so there isn't really anything new actually coming in.
For example, a few minutes ago while at my computer I received an email. My phone played the notification. I already have my Gmail inbox open in another tab, so I switch to it to see what the email is. I see the email, and click on it, then delete it, and at some point in all of that my phone plays the notification sound again, even though there isn't new email coming in).
At first I shrugged it off as possible coincidence, but it's too consistent. It happens every time. Bare in mind that I don't mean it happens every time I go to the Inbox tab. It only happens if I've received new email since my last visit to the inbox tab/page. I'll get the notification on my phone, then when I go to check it on my PC it plays on my phone a 2nd time.