Copy and paste of AutoMap Rules add that tileset a second time

[Seanba]

Describe the bug
When copying and pasting regions of tiles that include a tile from the Automap Rules tileset I've noticed that the Automap Rules tileset is added to the list of tilesets a second time.

To Reproduce
Steps to reproduce the behavior:

1. Open or create a map
2. Use the Map/Add Automapping Rules Tileset menu item to include that tileset
3. Place some of these tiles in a layer in your map
4. Use the Rectangular Select tool to select some of these tiles on your layer
5. Copy the selection
6. Paste the selection into your layer
7. See that you now have AutoMap Rules in your list of tilesets twice

Expected behavior
If the Automap Rules is already included then don't add it again.

Media

Specifications:

• OS: Windows 10
• Tiled Version: 1.9.1

[bjorn]

This appears to be caused by a slight difference in the path of the Automapping Rules Tileset. The one added by the action is loaded from ://automap-tiles.tsx, whereas the one that gets added after copy/paste is loaded from :/automap-tiles.tsx.

This also survives into the saved file:

 <tileset firstgid="22" source="://automap-tiles.tsx"/>
 <tileset firstgid="27" source=":/automap-tiles.tsx"/>

However, when loading this file the tilesets are "unduplicated", so some code somewhere realizes they are the same tileset, heh.