It's the fact that the tab doesn't know that you've handled them on another device. Not an android issue as such. And TBH I don't see a way of stopping it on the tab except by turning off notifications on it.
I don't know what apps you have this with, but with my tablet (also less used than my phone, and often inundated with notifications when I turn it on) I will receive notifications for new unread emails, but not for those that are read. Which is reasonable: the server tells the mail client which mails are read and which are unread, but it doesn't know which ones I've received notifications for on another device (it may know that another device has downloaded them, but it doesn't know that that device has notified me or that I've seen it). This also means that with one of my accounts both my phone and laptop will notify me of new mails unless I read it on one before the other downloads (i.e. it's the same with my laptop, not android, as with my different android devices).
With calendar apps if an appointment has a reminder set all devices it's synced with will notify me. So again my laptop and phone will both do so, and my tablet if it's on at the time (but it won't notify me of things that have been and gone in the meantime).
Without knowing what notifications you have I can't say what's reasonable. I'm just mentioning those as two examples of cases where I get multiple notifications and would expect to do so.
If you want an example of unreasonable notifications, then Facebook is exhibit number 1 (it does know exactly what you've done on any device, but is quite happy to notify you about something you read 2 hours ago on a different device or even using a different app. They don't care as long as it gets you to open their app again).
