`bst build` appears to build same module twice

I noticed bst 1.6.1 is building webkitgtk.bst twice at the same time:


That's twice the WebKit I had bargained for! I'm not certain, but I think it is not just a visual error: I think it really is building WebKit twice at once, because I see two separate chains of bst processes in gnome-control-center, each working on webkitgtk:



I attached two screenshots of that to show the tooltips for the two different bwrap processes that you can see are both working on webkitgtk.bst. Another clue that it's really doing a second build: my desktop was thrashing, which doesn't normally happen when building WebKitGTK via buildstream.

This is, sadly, not reproducible at all. After killing buildstream with Ctrl+C and restarting the build, it works fine, building webkitgtk.bst only once.

Start by reviewing the `bst build` entry point and the process scheduling behavior described in the issue. Try to reproduce the simultaneous `webkitgtk.bst` builds using the reported screenshots and process observations; done means confirming and preventing duplicate builds, with a regression test if the relevant test location can be identified.

Written by the indexing model from the issue text.