http://trac.webkit.org/changeset/57328 (bug 36899) changed DumpRenderTree to use a DUMPRENDERTREE_TEMP environment variable for storing databases, etc., so that concurrent instances of DRT won't stomp on each other. But this support was never added to Windows. We need to support this on Windows if we want to run multiple copies of DRT in parallel.
It looks like WebKit on Windows uses either CSIDL_LOCAL_APPDATA or CSIDL_APPDATA (depending on context) to store database and URL cache information.
See Bug 116729 for an implementation that uses CFPreferences to store the current application's desired storage paths. This is the technique being used in the OS X port, which seems to work properly in a concurrent run environment.
For now, we will use new-run-webkit-tests hard-coding it to use a single worker. This will ensure we don't have any problems running concurrently.
As a second step we will loosen this restriction once we've determined that the changes on Bug 116729 resolve the problem.
Fixed by use of separate environments per build process (Bug 116729).