Currently Hudson does not allow multiple builds in parallel for the same
project. However when a project is parameterized, this IS possible.

This is very dangerous, since it's perfectly possible that two builds will be
started on the same workspace. Imagine your workspace being updated by one build
while the other one is still doing the build. You might not even notice
something wrong has happened but your build results would be completely bogus.

The current workaround is to make sure you do only one parallel build per node.

The immediate solution is to disable parallel builds in the core, but this
requires implementing equals() and hashcode() differently on ALL Queue.Task
implementers.

The final solution is to implement decent locking, and fix all other places in
Hudson that depend on only having one build running.