In the build phase, we allow the same flag choice to occur multiple times.
This makes it easy to handle the situation where the same flag occurs
several times in the condition tree, and hence new goals and dependencies
might be introduced depending on the choice.

Previously, we have ensured during validation that repeated flag choices
are consistent. This behaviour has now been replaced by the new approach
to collapse repeated flag choice nodes completely during validation.

The advantage is that the tree is less deep, and that the trace output looks
less strange. Repeated flag choices are no longer seen, which I think avoids
confusion.