I have a multi-configuration project that is configured to run a job with different JDK versions that is configured as shown below:
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Source Code Management
Rational Team Concert (RTC)
RTC repository connection (julianpayne@fr.ibm.com@https://rtcdemeter.francelab.fr.ibm.com:9443/jazz)

Just accept and fetch from a build workspace
Build workspace

Configuration Matrix
JDK
IBM-JDK-7 Oracle-JDK-7 IBM-JDK-6

The problem seems to come from the fact that the "top-level" build:
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Started by user julianpayne@fr.ibm.com
Building on master in workspace /data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace
RTC : checkout...
RTC Checkout : Source control setup
RTC Checkout : Accepting changes into workspace "JenkinsRunAsfTests10" ...
RTC Checkout : Fetching files to fetch destination "/data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace" ...
RTC Checkout : Fetching Completed
Triggering IBM-JDK-7
Triggering Oracle-JDK-7
IBM-JDK-7 completed with result FAILURE
Oracle-JDK-7 completed with result FAILURE
Finished: FAILURE

Then the individual builds try to do a checkout in their private workspace:
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Started by upstream project "RunAsfTests10" build number 2
originally caused by:
Started by user julianpayne@fr.ibm.com
Building on master in workspace /data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace/jdk/IBM-JDK-7
RTC : checkout...
RTC Checkout : Source control setup
RTC Checkout : Accepting changes into workspace "JenkinsRunAsfTests10" ...
FATAL: RTC : checkout failure: Cannot create sandbox at /data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace/jdk/IBM-JDK-7 because one already exists at /data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace
com.ibm.team.filesystem.client.FileSystemStatusException: Status ERROR: com.ibm.team.filesystem.client code=0 Cannot create sandbox at /data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace/jdk/IBM-JDK-7 because one already exists at /data/demeter/dev/jenkins-build/jobs/RunAsfTests10/workspace null
at com.ibm.team.filesystem.client.internal.copyfileareas.CopyFileAreaManager.createCopyFileArea(CopyFileAreaManager.java:309)
at com.ibm.team.filesystem.client.internal.copyfileareas.CopyFileAreaManager.createCopyFileArea(CopyFileAreaManager.java:1)
at com.ibm.team.filesystem.client.internal.SharingManager.register(SharingManager.java:1002)
at com.ibm.team.build.internal.hjplugin.rtc.RepositoryConnection.checkout(RepositoryConnection.java:363)