SOLUTION FOUND!! Or so it seems, and this is at the user level, with ZWO perhaps not needing to update the software for the ASIAir Pro or Plus, unless they know of a vulnerability in the software to what I've found as the cause, and if they can then implement a fix so that the problem can be avoided altogether.
Firstly, Klaus...I'm sorry to say that for your Nikon Z 50, the problem may be due to other reasons, as what I've observed was always pairs of consecutive, valid FITs files with correct pixel dimensions and fully usable, just duplicates.
So, what is the solution? I accidentally stumbled upon this tonight, when following an update to the latest 1.8.3 firmware and running a test of it, I did NOT see the duplicate file problem...not at first. But, later I did, and I realized the one change I had made was to install the 2nd memory card of my Nikon D850, which I'd happened to noticed was still in my card reader.
Sure enough....verifying this for both my D850 which uses an XQD and SDXC memory card, as well as my D7100 which uses two SDXC memory cards, if both cards are installed in the camera, then during an Autorun sequence for the ASIAir Pro/Plus, sequential duplicates of each image captured will be written and saved as a FIT file. And again, when that occurs and the "missing" image is not used for display to the app screen, that image will get "backlogged" and eventually displayed, by which time it is effectively stale data. So, Nikon DSLR and Mirrorless camera users...if your camera supports dual memory cards, DO NOT USE BOTH CARDS. Use only a single card and this problem should go away if you've ever experienced it.
It may well be the problem hasn't been seen or reported often because maybe not so many owners of cameras with dual card support actually use both slots. While I've only heard from one other user of a Nikon D810 having seen this problem as I described it initially, it seems the type of thing that could also be common to other brands of DSLRs or mirrorless cameras with dual memory card support. So, regardless of brand, I suggest only one memory card be used with the camera while still continuing to use either the internal memory (ASIAir Plus) or MicroSD card (ASIAir Pro), or external USB storage with either device as your "backup", should you be one who generally uses your raw files for your astrophotography post-processing. Meanwhile, you've got the FIT files as a backup to the raw. Of course the reverse is true as well...if you use FITs files because they may be better utilized for the extra information they provide for astrophotography processing software, then you'll have your raw files for backup if needed.
Again....if your camera supports dual memory cards, do not use but one.