1. Right click an effect in Content Library --> Effects
2. Click "Add an instance" (or drag-and-drop the effect on to a model)

1. 1. The effect is added to the model 
2. Now right click the same effect and select "Remove from project"
   1. Effect is removed from the project
3. Now right click the same effect and select again "Add an instance" (or drag-and-drop the effect on to a model)
   1. The effect is added again to the model, but it is now duplicated

If the user keeps doing the steps as mentioned above, the effect will keep duplicating.

See the video for a demonstration of this bug: 

qtdesignstudio_sxk1nlHRKt.mp4