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You can use this code to prevent a program from being started again when it already runs:

<vip>class facts
    okToRunMutex : mutex := erroneous.
class predicates
    okToRun : () determ.
clauses
    okToRun() :-
        okToRunMutex := mutex::createNamed("SomeStrangeName", false()),
        not(multiThread_native::wait_timeout = okToRunMutex:wait(0)).

clauses
    run():-
        if okToRun() then
            TaskWindow = taskWindow::new(),
            TaskWindow:show()
        end if.</vip>

You are supposed to use some strange name instead of "SomeStrangeName". Microsoft has this to say about the subject: 

''If you are using a named mutex to limit your application to a single instance, a malicious user can create this mutex before you do and prevent your application from starting. To prevent this situation, create a randomly named mutex and store the name so that it can only be obtained by an authorized user. Alternatively, you can use a file for this purpose. To limit your application to one instance per user, create a locked file in the user's profile directory.''

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View source for Preventing two instances of a program to run simultaneously

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