Hi Arno, I have both good and bad news. Bad news mostly for me, good for most people, but I think it should be here, so maybe it's more common thing, so latest update on this:
on my local dev machine I run Windows and have WAMP installed. Pretty default config for server, nothing special. I tested 3 different versions of php (7.0.x, 7.2.x and 7.3.x) all give me same result: doubling batch requests (no matter which options I choose). Here and there it happens that it doensn't, but 99% it makes 2 instances. What I tested more is that it does the SAME THING WITH CATEGORIES! So - batch process category -> makes 2 categories. Also, I tested 3.2.69 version - SAME THING HAPPENS.
Good news is that I tested it on our production server here, Linux blahblah, more serious - ALL WORKS FINE! My guess is that most people are not using WAMP or integrated solutions except for developing purposes, so it won't be so common. Question still remains - why it happens. I'll try to figure it out somehow...
Cheers, thank you for your excellent work.