Prune duplicate elements in resources

Prior to this commit a list with potential duplicates is generated
and verified when loading the `junit-platform.properties` file.

Addresses #2207

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/LauncherConfigurationParameters.java

@@ -14,10 +14,11 @@
 import java.net.URL;
 import java.net.URLConnection;
 import java.util.Collections;
-import java.util.List;
+import java.util.HashSet;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Properties;
+import java.util.Set;
 
 import org.junit.platform.commons.logging.Logger;
 import org.junit.platform.commons.logging.LoggerFactory;
@@ -52,7 +53,7 @@ private static Properties fromClasspathResource(String configFileName) {
 
 		try {
 			ClassLoader classLoader = ClassLoaderUtils.getDefaultClassLoader();
-			List<URL> resources = Collections.list(classLoader.getResources(configFileName));
+			Set<URL> resources = new HashSet<>(Collections.list(classLoader.getResources(configFileName)));
 
 			if (!resources.isEmpty()) {
 				if (resources.size() > 1) {
@@ -61,7 +62,7 @@ private static Properties fromClasspathResource(String configFileName) {
 						resources.size(), configFileName));
 				}
 
-				URL configFileUrl = resources.get(0);
+				URL configFileUrl = resources.iterator().next(); // same as List#get(0)
 				logger.info(() -> String.format(
 					"Loading JUnit Platform configuration parameters from classpath resource [%s].", configFileUrl));
 				URLConnection urlConnection = configFileUrl.openConnection();