Not quite sure what happened here, but my dock now looks as follows:
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Try to delete the Dock Preference setting, open Terminal:
sudo rm ~/Library/Preferences/com.apple.dock.plist
Enter your password.
Reboot your computer. The dock should now be back to the original (out-of-box) settings.
had the same problem, tried all the solutions proposed using terminal and commands. Found the solution by just dropping the icon first on the desktop and then into the dock. Stayed there and was visible