I’m using the latest snapshot version of Tiled on Windows 7, but this issue has been around for a few versions (as long as I can remember actually). The situation is that I have a .json map, although the problem is also there with tmx files. This map has a number of exported tilesets. When opening the map, Tiled now picks a random number, and shows that many tabs in the tileset window, for a specific external tileset. This random number can differ for every tileset. As an example, I’ve added a screenshot from a map with just 3 external tilesets (animations, ground, rockWalls), which can also be seen in the .json source. But it shows multiple tabs. As far as I’ve noticed, this only happens with exported tilesets.
I can’t delete the tilesets from the extra tabs; the trash-bin icon is greyed-out. When using a tile from one of the extra tabs, the map acts as if I placed a tile from the original tab. So it’s not really a bug, but it is kind of annoying.