Experiencing a persistent issue in Ubuntu where the Shift key seems to trigger repetitive pasting of copied content in the terminal can be quite exasperating. This problem is typically associated with keyboard shortcuts and may have multiple underlying causes. Below, you'll find a set of steps to help you identify and potentially fix this vexing issue:
1. Terminal Preferences: Check your terminal emulator's preferences for conflicting or misconfigured keyboard shortcuts (e.g., Ctrl+Shift+V or Shift+Insert).
2. Clipboard Managers: Disable or configure any clipboard manager extensions or applications, as they can interfere with shortcuts.
3. Keyboard Layout and Keybindings: Ensure your keyboard layout is correct and check Ubuntu's system settings for shortcut conflicts.
4. Test with Different Terminal: Try alternative terminal emulators like xterm or Terminator to isolate the issue.
5. External Keyboards: If using an external keyboard, test with the laptop keyboard to rule out custom keybindings or firmware problems.
6. Reset Keyboard Shortcuts: If you suspect that a custom keyboard shortcut is causing the problem, you can try resetting your keyboard shortcuts to their default values. In Ubuntu, you can often reset keyboard shortcuts from the System Settings or by using the dconf or gsettings commands [3][4][5][6].
The Steps to reset using dconf or gsettings command:
* List Keyboard Shortcuts: First, use the dconf command to list the keyboard shortcuts for the GNOME Terminal. This command will display the available keybindings for the GNOME Terminal:

        dconf list /org/gnome/terminal/legacy/keybindings/
        OR 
        gsettings list-recursively | grep -i org.gnome.Terminal.Legacy.Keybindings

* Identify the Problematic Keybinding: Look for the keybinding that corresponds to the Shift key pasting behavior. It may be named something like "paste" or "insert."
* Reset the Problematic Keybinding: Once you identify the problematic keybinding, you can reset it to its default value using dconf. For example, to reset the "paste" keybinding in the GNOME Terminal. This command will reset the "paste" keybinding in the GNOME Terminal to its default value.

        dconf reset /org/gnome/terminal/legacy/keybindings/paste
       OR 
        gsettings reset org.gnome.Terminal.Legacy.Keybindings:/org/gnome/terminal/legacy/keybindings/ paste

7. Update or Reinstall Drivers: Keep terminal and keyboard drivers up-to-date or consider reinstalling them to resolve problems.
8. Malware Check: Scan your system for malware or keyloggers that might interfere with keyboard input, although this is less common.
9. Check Hardware: Physically inspect your keyboard for stuck keys or damage, as hardware issues can lead to unintended keypresses.
10. Community Resources: Consult Ubuntu forums and documentation for solutions if the issue persists. Others may have encountered and resolved similar keyboard problems.

If the problem still persists after trying these steps, you may want to consider seeking help from the Ubuntu community or technical support to diagnose and address the issue further.