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Just recently updated to the latest version of Illustrator 2021 (previously on 2020). Using Windows 10 on fairly powerful machine.
I use Alt + Shift alonside the left mouse (held down) to duplicate objects but also keeping them perfectly in line with the original. This however does not work correctly if Smart Guides are ON.
It will appear to be working then snap down to align to an object of its choice. If you keep dragging the mouse pointer it will jump about aligning one object after another.
This is a really annoying glich as I use this method constantly throughout my day - it works as normal if Smart Guides are set to OFF.
Sorry to hear about the annoying glitch.
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