Add to panel from the menu can add new panel-launcher applets ad infinitum in some circumstances.

 * Cinnamon version 4.0.9
 * Distribution - Mint 19.1 - issue originally raised on Mint Forum by an Arch user.
 * Graphics hardware *and* driver used
Graphics:
  Device-1: Intel 2nd Generation Core Processor Family Integrated Graphics 
  driver: i915 v: kernel 
  Display: x11 server: X.Org 1.19.6 driver: modesetting unloaded: fbdev,vesa 
  resolution: 1366x768~60Hz 
  OpenGL: renderer: Mesa DRI Intel Sandybridge Mobile v: 3.3 Mesa 18.2.2 
 * 64 bit

Issue
In limited circumstances using add to panel from the menu right-click context menu can result result in additional panel launcher applets being added without the target application be added as a panel-launcher.

Steps to reproduce

Starting from a default Cinnamon 4.0.9 layout (modern in Mint), right click on the GWL and remove. Right click on the panel > Add applets to the panel > Select and add a panel launcher applet. This works fine up to this point

Navigate to the menu and attempt to add an application to the panel. Observe a 2nd panel launcher applet being created with the default launchers present but not the target application launcher. Repeat and observe a 3rd etc.

Restart Cinnamon with Ctrl-Alt-Esc and retry. Now the application will be added to the most recent panel launcher apple and all is OK.

The alternative workflow, Starting from a default Cinnamon 4.0.9 layout (modern in Mint), right click on the GWL and remove, navigate to menu and attempt to add an application to the panel also has the same result.

Expected behaviour
A cinnamon restart should not be necessary for new panel launcher applet to be recognised as a target for add to panel.

Other information

This issue also manifests attempting the reverse. Start with the Mint 19.1 traditional layout, remove the panel launcher applet, add a grouped-window-list-applet. On attempting add to panel a new panel-launcher is created (although this time with the default launchers and the target application). Again a cinnamon restart is needed to avoid this behaviour and add to panel to result in a new GWL pinned app.

Reproduce the issue through the menu's Add to panel workflow after removing or adding the panel-launcher and grouped-window-list applets. Start by tracing the menu and panel applet handling; no files or tests are named in the report. Done means adding an application targets the existing applet without creating duplicates or requiring a Cinnamon restart.