Lomand

I see you lost your interest in this case.
…
I’ve just created a blank wordpress site with only one plugin installed:Social Networks Auto-Poster. I did it to become certain, that missing VK image is not problem caused my particular setup.
So, as I expected, auto post still does not operate properly.
If you want do dig deeper in this problem, I can provide you admin access
to the blank site.

Yes, I’m pretty sure about that.

Of course, but it was text with attached link (which available for PRO users only, by the way) and not image post.
Here’s what I’m talking about

Oh, it seems I messed up.
You see, I use wordpress-multisite, and when I disable plugin in network menu, it usually disables on each network site. For some reason, something went wrong with PBP Upload increase upload limit plugin, it was still enagled on some sites. When I disabled it manually in the site plugin menu, EWWW IO began to work properly, no more duplicates or errors, what a relief…
Thanks for your support!

Plugin List.
I’m sure that problem is not plugin interference. I’d tried to disable them when I had duplicated images and had no luck.
…
EWWW Image Optimizer Settings (when images duplicates)
Duplication demo gif
And here is a new issue. If “delete original” is activated, but optipng and pngout is active, I have “HTTP error” without duplication.
Demo gif
If ptipng and pngout are disabled, everything operates normally.
…
Image used in the demos.

I finaly resolve the problem.
As you said, I’d disabled optipng and pngout in the plugin options, and everything works fine right now.
My site contains photographic images mostly, so I guess this changes are for the best.
Thanks for fast and accurate responce!