Yes, for me it would be #1

@willdaly Now that Zotero 7 beta fully supports split view, opening multiple tabs is likely less relevant. Does it?

The problem is exactly the same in split view. The expected behaviour for me would be to keep the same position (and zoom) in all tabs or split views after reloading a tab or restarting Zotero as it was before reloading or restarting.

If I am using a split view to look at the main text at the top while looking at the references in the bottom, I just want to keep it like this after reopening Zotero. But currently, after reloading the tab or restarting Zotero, both split views are set to the position of the "main" or "primary" view (top here).

Duplicate tabs are still very useful for looking at different positions of the same PDF file in full screen, as mentioned above. I use it especially in 2 different windows on two monitors to do the same as the split view with more space. At the moment, when the tab is reloaded, it uses the position of the tab where the position was last moved, when I would expect to get back to the same as it was before.
[It seems that tabs opened in a new window never reload, so only the position in the main window would be reset.
Zotero cannot restore multiple windows after restart yet, but the question will still come up when this is implemented.]

you close both tabs on this device and reopen one later, which position should it use? [...]
Given that additional tabs are likely to be used for viewing references/footnotes, I guess I'd be inclined to go with (1) — that the first tab you open of given PDF is the primary tab, and only primary tabs update the synced page position, while other tabs preserve their position locally.

For me, the idea of reopening the "primary tab" (first opened) is problematic:
A1) I could easily decide to open the first and second tab of the same PDF at the same time, and decide to use the second one as the main tab I want to get reopened later.
A2) Tabs can be reorganized. So a secondary tab can easily become the main tab.
A3) How should I differentiate the primary tab from secondary tabs when closing them to keep only one that will be reopened later? There is no way to distinguish which one was "the first tab you opened of that PDF". Especially if you keep this "primary tab" property after restarting Zotero.
A4) If you open a PDF file in a new window from the library, nothing will tell you if the PDF file is already opened in the main Zotero window. So you have no way of knowing if you have actually opened a "secondary" tab.
A5) If you close a secondary tab last, you expect that Ctrl+Shift+T gets you back to the position of that secondary tab that you have just closed. In that case, do you decide that this newly opened tab becomes a "primary tab" or remains a "secondary tab"?

Considering these issues, I think that option 3 is better ("The last tab that you closed").
B1) It is not perfect, as you can forget about other duplicate tabs. But at least I think that it is the easiest option to control in a logical way which position to keep when closing duplicate tabs.
B2) It makes sense to close the references first before closing the main text. So this option is still compatible with the expected outcome in most cases.
B3) If I open several duplicates, close them all exept one and continue working on that one, the only logical outcome I would expect is that the position of that tab is saved.
B4) When the tabs history is implemented, each saved record should probably also save the position in that tab. In that way, it will be easier to recover the reading position after closing a tabs even if the default is not what the user wanted.