Don't call CloseHandle() twice on Windows (as it causes crashes, or unexpected behavior). This would happen with the following test case:

ProcessHandle handle = Process.launch(...);
handle.kill();
Then as handle gets out of scope, ~ProcessHandle would call CloseHandle() on an already closed handle.

Foundation/include/Poco/Process.h

@@ -237,7 +237,7 @@ public:
 		/// Waits for the process specified by handle to terminate
 		/// and returns the exit code of the process.
 		
-	static void kill(const ProcessHandle& handle);
+	static void kill(ProcessHandle& handle);
 		/// Kills the process specified by handle.
 		///
 		/// This is preferable on Windows where process IDs

Foundation/include/Poco/Process_UNIX.h

@@ -84,7 +84,7 @@ public:
 		Pipe* outPipe, 
 		Pipe* errPipe,
 		const EnvImpl& env);
-	static void killImpl(const ProcessHandleImpl& handle);
+	static void killImpl(ProcessHandleImpl& handle);
 	static void killImpl(PIDImpl pid);
 	static void requestTerminationImpl(PIDImpl pid);
 

Foundation/include/Poco/Process_VMS.h

@@ -85,7 +85,7 @@ public:
 		Pipe* errPipe,
 		const EnvImpl& env);
 	static int waitImpl(PIDImpl pid);
-	static void killImpl(const ProcessHandleImpl& handle);
+	static void killImpl(ProcessHandleImpl& handle);
 	static void killImpl(PIDImpl pid);
 	static void requestTerminationImpl(PIDImpl pid);
 };

Foundation/include/Poco/Process_VX.h

@@ -86,7 +86,7 @@ public:
 		Pipe* outPipe, 
 		Pipe* errPipe,
 		const EnvImpl& env);
-	static void killImpl(const ProcessHandleImpl& handle);
+	static void killImpl(ProcessHandleImpl& handle);
 	static void killImpl(PIDImpl pid);
 	static void requestTerminationImpl(PIDImpl pid);
 };

Foundation/include/Poco/Process_WIN32.h

@@ -62,6 +62,7 @@ public:
 	UInt32 id() const;
 	HANDLE process() const;
 	int wait() const;
+	void closeHandle();
 
 private:
 	HANDLE _hProcess;
@@ -89,7 +90,7 @@ public:
 		Pipe* outPipe, 
 		Pipe* errPipe,
 		const EnvImpl& env);
-	static void killImpl(const ProcessHandleImpl& handle);
+	static void killImpl(ProcessHandleImpl& handle);
 	static void killImpl(PIDImpl pid);
 	static void requestTerminationImpl(PIDImpl pid);
 	static std::string terminationEventName(PIDImpl pid);

Foundation/include/Poco/Process_WIN32U.h

@@ -62,6 +62,7 @@ public:
 	UInt32 id() const;
 	HANDLE process() const;
 	int wait() const;
+	void closeHandle();
 
 private:
 	HANDLE _hProcess;
@@ -89,7 +90,7 @@ public:
 		Pipe* outPipe, 
 		Pipe* errPipe,
 		const EnvImpl& env);
-	static void killImpl(const ProcessHandleImpl& handle);
+	static void killImpl(ProcessHandleImpl& handle);
 	static void killImpl(PIDImpl pid);
 	static void requestTerminationImpl(PIDImpl pid);
 	static std::string terminationEventName(PIDImpl pid);

Foundation/include/Poco/Process_WINCE.h

@@ -62,6 +62,7 @@ public:
 	UInt32 id() const;
 	HANDLE process() const;
 	int wait() const;
+	void closeHandle();
 
 private:
 	HANDLE _hProcess;
@@ -89,7 +90,7 @@ public:
 		Pipe* outPipe, 
 		Pipe* errPipe,
 		const EnvImpl& env);
-	static void killImpl(const ProcessHandleImpl& handle);
+	static void killImpl(ProcessHandleImpl& handle);
 	static void killImpl(PIDImpl pid);
 	static void requestTerminationImpl(PIDImpl pid);
 	static std::string terminationEventName(PIDImpl pid);

Foundation/src/Process.cpp

@@ -200,7 +200,7 @@ int Process::wait(const ProcessHandle& handle)
 }
 
 
-void Process::kill(const ProcessHandle& handle)
+void Process::kill(ProcessHandle& handle)
 {
 	killImpl(*handle._pImpl);
 }

Foundation/src/Process_UNIX.cpp

@@ -218,7 +218,7 @@ ProcessHandleImpl* ProcessImpl::launchByForkExecImpl(const std::string& command,
 }
 
 
-void ProcessImpl::killImpl(const ProcessHandleImpl& handle)
+void ProcessImpl::killImpl(ProcessHandleImpl& handle)
 {
 	killImpl(handle.id());
 }

Foundation/src/Process_VMS.cpp

@@ -137,7 +137,7 @@ ProcessHandleImpl* ProcessImpl::launchImpl(const std::string& command, const Arg
 }
 
 
-void ProcessImpl::killImpl(const ProcessHandleImpl& handle)
+void ProcessImpl::killImpl(ProcessHandleImpl& handle)
 {
 	killImpl(handle.id());
 }

Foundation/src/Process_VX.cpp

@@ -89,7 +89,7 @@ ProcessHandleImpl* ProcessImpl::launchImpl(const std::string& command, const Arg
 }
 
 
-void ProcessImpl::killImpl(const ProcessHandleImpl& handle)
+void ProcessImpl::killImpl(ProcessHandleImpl& handle)
 {
 	throw Poco::NotImplementedException("Process::kill()");
 }

Foundation/src/Process_WIN32.cpp

@@ -56,9 +56,17 @@ ProcessHandleImpl::ProcessHandleImpl(HANDLE hProcess, UInt32 pid):
 
 ProcessHandleImpl::~ProcessHandleImpl()
 {
-	CloseHandle(_hProcess);
+	closeHandle();
 }
 
+void ProcessHandleImpl::closeHandle()
+{
+	if (_hProcess)
+	{
+		CloseHandle(_hProcess);
+		_hProcess = NULL;
+	}
+}
 
 UInt32 ProcessHandleImpl::id() const
 {
@@ -228,17 +236,19 @@ ProcessHandleImpl* ProcessImpl::launchImpl(const std::string& command, const Arg
 }
 
 
-void ProcessImpl::killImpl(const ProcessHandleImpl& handle)
+void ProcessImpl::killImpl(ProcessHandleImpl& handle)
 {
+	if (handle.process()) 
+	{
 		if (TerminateProcess(handle.process(), 0) == 0)
 		{
-		CloseHandle(handle.process());
+			handle.closeHandle();
 			throw SystemException("cannot kill process");
 		}
-	CloseHandle(handle.process());
+		handle.closeHandle();
+	}
 }
 
-
 void ProcessImpl::killImpl(PIDImpl pid)
 {
 	HANDLE hProc = OpenProcess(PROCESS_TERMINATE, FALSE, pid);

Foundation/src/Process_WIN32U.cpp

@@ -57,9 +57,17 @@ ProcessHandleImpl::ProcessHandleImpl(HANDLE hProcess, UInt32 pid):
 
 ProcessHandleImpl::~ProcessHandleImpl()
 {
-	CloseHandle(_hProcess);
+	closeHandle();
 }
 
+void ProcessHandleImpl::closeHandle()
+{
+	if (_hProcess)
+	{
+		CloseHandle(_hProcess);
+		_hProcess = NULL;
+	}
+}
 
 UInt32 ProcessHandleImpl::id() const
 {
@@ -234,17 +242,19 @@ ProcessHandleImpl* ProcessImpl::launchImpl(const std::string& command, const Arg
 }
 
 
-void ProcessImpl::killImpl(const ProcessHandleImpl& handle)
+void ProcessImpl::killImpl(ProcessHandleImpl& handle)
 {
+	if (handle.process())
+	{
 		if (TerminateProcess(handle.process(), 0) == 0)
 		{
-		CloseHandle(handle.process());
+			handle.closeHandle();
 			throw SystemException("cannot kill process");
 		}
-	CloseHandle(handle.process());
+		handle.closeHandle();
+	}
 }
 
-
 void ProcessImpl::killImpl(PIDImpl pid)
 {
 	HANDLE hProc = OpenProcess(PROCESS_TERMINATE, FALSE, pid);

Foundation/src/Process_WINCE.cpp

@@ -57,9 +57,17 @@ ProcessHandleImpl::ProcessHandleImpl(HANDLE hProcess, UInt32 pid):
 
 ProcessHandleImpl::~ProcessHandleImpl()
 {
-	CloseHandle(_hProcess);
+	closeHandle();
 }
 
+void ProcessHandleImpl::closeHandle()
+{
+	if (_hProcess)
+	{
+		CloseHandle(_hProcess);
+		_hProcess = NULL;
+	}
+}
 
 UInt32 ProcessHandleImpl::id() const
 {
@@ -158,14 +166,17 @@ ProcessHandleImpl* ProcessImpl::launchImpl(const std::string& command, const Arg
 }
 
 
-void ProcessImpl::killImpl(const ProcessHandleImpl& handle)
+void ProcessImpl::killImpl(ProcessHandleImpl& handle)
 {
+	if (handle.process()) 
+	{
 		if (TerminateProcess(handle.process(), 0) == 0)
 		{
-		CloseHandle(handle.process());
+			handle.closeHandle();
 			throw SystemException("cannot kill process");
 		}
-	CloseHandle(handle.process());
+		handle.closeHandle();
+	}
 }