I had the same problem and I was placing solar panels on a large roof space.

Best way to find duplicates is to edit the component and make the face or shape colour and then change the opacity to half. any duplicates will then be staked and show on the model as darker. then you can simply delete the dark components.

I've tried really hard to sort a similar problem by searching the forums for answers but still can't do it. I have Auto CAD 3D spot heigh data and need to convert it into a 3d landscape terrain. I can't find any plugins that work on my Mac with Sketchup Pro 2016. Any help would be greatly appreciated.