fixes #195 Config instances can only be used once

src/main/java/teetime/framework/Configuration.java

@@ -25,6 +25,16 @@ package teetime.framework;
  */
 public class Configuration extends AbstractCompositeStage {
 
+	private boolean executed;
+
+	boolean isExecuted() {
+		return executed;
+	}
+
+	void setExecuted(final boolean executed) {
+		this.executed = executed;
+	}
+
 	protected Configuration() {
 		// protected ctor to prevent direct instantiation.
 	}

src/main/java/teetime/framework/Execution.java

@@ -113,6 +113,10 @@ public final class Execution<T extends Configuration> implements UncaughtExcepti
 	public Execution(final T configuration, final boolean validationEnabled, final IExceptionListenerFactory factory) {
 		this.configuration = configuration;
 		this.factory = factory;
+		if (configuration.isExecuted()) {
+			throw new IllegalStateException("Configuration was already executed");
+		}
+		configuration.setExecuted(true);
 		if (validationEnabled) {
 			validateStages();
 		}

src/test/java/teetime/framework/ExecutionTest.java

@@ -198,4 +198,11 @@ public class ExecutionTest {
 
 	}
 
+	@Test(expected = IllegalStateException.class)
+	public void executeConfigOnlyOnce() {
+		NameConfig configuration = new NameConfig();
+		new Execution<NameConfig>(configuration);
+		new Execution<NameConfig>(configuration); // do not execute, but just initialize the execution
+	}
+
 }