I appreciate Mark's thinking on this, but I don't use CC libraries and I mostly build in CMYK. This looks and behaves just like the issues I have been having with styles duplicating endlessly for 10 YEARS' worth of Illustrator versions. No amount of deleting preferences, reinstalling, buying new computers and installing, creating new user profiles and installing, or anything else has ever solved it.

I work in macOS and periodically run into this problem. I have created a graphic style for stroked line with weight and global color. The style cloning bug occurs randomly. Sometimes I copy an object with a style and the style is duplicated, sometimes the style is not duplicated. This is annoying.

Mark, thanks for the investigation. Feels like it :(

One cause could be color. If your style is using a "Global" color that was originally made in a CMYK document, and re-used via CC Library in an RGB document, this color is conflicting with itself every time it's copied back in, hence duplicates in Styles. If you convert the color to show its values, and redefine the style, copy/paste no longer causes duplicates.

This issue has persisted for at least a year and it is unnerving users. Styles are something so elementary that it makes me wonder what Adobe's attitude is towards fixing this bug.

It happens with me all times

same here no matter if on Mac or Win - issue is file related only
tested with 2020, 2021 offical release and even PR v25.4

does not duplicate if option dragged
does not appear when object has been unlinked to graphic style and new graphic style has been created and applied to same object
does not spread if objects with such graphic styles are copied to other files

check yourself using the following examples:

It also happens when you option-drag!

I have the same problem. It is quite serious because I am working on a file with over 1000 icons and each icon exists for dark mode and for light mode. To tweak icons as the application gets developed on, I need to make fine adjustments to colours so the icons look good on the given backgrounds. With styles getting duplicated as objects get duplicated (usually not by copy/paste but by option-drag on Mac), I lose track of applied styles.

macOS Big Sur 11.2.1
Illustrator 25.2

Under certain conditions, copy-pasting objects will duplicate the graphic style.
(CMYK documents, styles with no fill)

To replicate the issue:
Create a new CMYK document.
Create an object.
Starting with the Default style, assign the Fill to None, then create a new graphic style.
Assign this style to the object.
Save the document and close it (no need to close Illustrator).
Open the document again.
Copy the object then paste it with any pasting command. A duplicate of that style will appear in the Styles panel. Pasting on All Artboards will create as many copies as there are artboards in the document. Alt-dragging works fine.

This is not a new bug, it’s been there for as long as I can remember.

I have experienced the same issue: I have some graphic styles applied to some objects. Any time I duplicate these objects, I get my graphic styles also duplicated.

It's annoying and a performance killer.

It is very annoying.