I can't replicate it in survival, but it probably shouldn't happen at all, regardless of gamemode.

To reproduce, move an item directly down in your inventory and sort it. Sometimes it gets duped.

Does this happen when sorting using the button and not the keybinding/middle click?

Both the button and the key/mouse binding cause it to happen.

Creative inventory or all inventories?

Only the Creative inventory.

The player inventory section of the creative inventory is a bit wonky when it comes to things like this, might have to set up a separate sorting condition for it. I’ll look into it, thank you for the bug report