Steps to reproduce:
1) write -Dfoo=true in system configuration "Global MAVEN OPTS"
2) Execute a maven job (see the definition passing by in the console output
3) verify <MavenOpts>-Dfoo=true</MavenOpts> in jobs/<jobname>/config.xml
4) clean system configuration "Global MAVEN OPTS"
5) Re-execute the same maven job (definition is still there)
6) verify <MavenOpts>-Dfoo=true</MavenOpts> in jobs/<jobname>/config.xml

for a correct behavior the mavenOpts tag in config.xml should be cleared.