If both Jenkins and a job define the same environment variable, the job definition will be ignored. It would be preferable to have the job definition supersede the Jenkins definition, so that individual jobs could choose to use a value other than the system-wide default.

Below is an example of this occurring. The FOO environment variable is not defined by Jenkins. The INSTALLERS_DIR variable is already defined by Jenkins to be 'V:\INSTALLERS':

-----------------------------------------------------------------------
[EnvInject] - Injecting as environment variables the properties content
'FOO=Bar
INSTALLERS_DIR=Bar'

[EnvInject] - Unset 'PROMPT' variable.
Building on master
[workspace] $ cmd /c call C:\DOCUME~1\ciavonne\LOCALS~1\Temp\hudson1081703287611836097.bat

FOO=Bar
INSTALLERS_DIR=V:\INSTALLERS

Finished: SUCCESS
-----------------------------------------------------------------------

As can be seen in the example, the FOO variable is injected correctly but the INSTALLERS_DIR variable retains the original value.