Created attachment 147557 [details]
Example project that demonstrates the problem.

In my project I need to perform some preprocessing of the compiled output, so I added a task in build.xml to perform it, like so.

    <target name="-post-compile">
        <java jar="PreprocessingUtility.jar" output="preprocessing.txt" />
    </target>

This runs fine, and does what I want it to, however every line of STDOUT that occurs after this is displayed twice. In particluar, if I run/debug my application in Netbeans each line of the application's output is duplicated. This can make it more difficult to follow what is going on in the the application.

I have attached a pair of simple projects that demonstrate the problem. Both the preprocessing utility and the main application do nothing but print a single line. The preprocessing utility was built, and it's jar copied to the main application directory, where Ant is looking for it. Then when running the main application in Netbeans I get the following output:

    ant -f C:\\Users\\djpeaco\\Documents\\TAVS\\MainApplication -Dnb.internal.action.name=run run
    init:
    Deleting: C:\Users\djpeaco\Documents\TAVS\MainApplication\build\built-jar.properties
    deps-jar:
    Updating property file: C:\Users\djpeaco\Documents\TAVS\MainApplication\build\built-jar.properties
    compile:
    run:
    Hello from the main application
    Hello from the main application
    BUILD SUCCESSFUL (total time: 1 second)

When using ant to running the same task from the commandline, the problem doesn't occur:
    Unable to locate tools.jar. Expected to find it in C:\Program Files (x86)\Java\jre8\lib\tools.jar
    Buildfile: C:\Users\djpeaco\Documents\MainApplication\build.xml
...
    init:

    -deps-jar-init:
       [delete] Deleting: C:\Users\djpeaco\Documents\TAVS\MainApplication\build\built-jar.properties

    deps-jar:

    -warn-already-built-jar:
    [propertyfile] Updating property file: C:\Users\djpeaco\Documents\TAVS\MainApplication\build\built-jar.properties
...
    -compile-depend:

    -do-compile:

    -post-compile:

    compile:

    run:
         [java] Hello from the main application

    BUILD SUCCESSFUL
    Total time: 2 seconds

I've tried setting fork="true" on the java task, as well as trying several other options, but they don't have any effect. However, if I eliminate the output redirection of the java task the problem goes away. As a work around I can have my preprocessing utility write to a file instead of STDOUT.