Based on the following steps from T76738: Assert failure undoing view layer creation.

Pasting older files into newer blender versions is adding a Tint brush.

In T76738#931299, @Sergey Sharybin (sergey) wrote:

Is the duplicate brush a root cause of the initial report?

Here are the steps to reproduce the duplicate brushes:

1. Open Blender 2.80 --factory-startup
2. Select everything (A in viewport, Cube, Camera, Lamp)
3. Ctrl-C
4. Open latest master --factory-startup
5. Delete everything
6. Ctrl-V
7. for b in bpy.data.brushes: print(b.name) will show duplicate Tint brush

Note that adding data-blocks in versioning code isn't something we do (except for screen data), and I think it would be better to avoid it.