The reason you are getting multiple pushes is simply due to the fact you opted into one of your staging/preview sites.
For example, say I have a dummy site: https://aimtell.info which I've added to my dashboard. You'd be able to opt into that site since it's a normal site. Now, I'm considering switching themes because, well, my dummy site looks terrible. So, I go into Shopify to pick a new theme and then grab the share preview link.
Now, if I view that preview it's actually a completely different domain/site but still has the original aimtell code for the aimtell.info site. As a result, viewing it will trigger the optin prompt for push for aimtell.info. If I subscribe when prompted on that one it'll add me as a subscriber under the initial site aimtell.info. At that point, when sending a push I'll get a push for both aimtell.info as well as shopifypreview.com.
As a result of all this, it'll seem as though you're getting more than one push when in reality, you're still only sending 1 push.
P.S. To prevent confusion, you can certainly make a segment targeting those who viewed those "shopifypreview.com" pages and then delete the subscribers.