SPIGOT-893: Fix bridge method registering generic listeners twice

By: Jonas Konrad <me@yawk.at>

paper-api/src/test/java/org/bukkit/event/SyntheticEventTest.java

@@ -0,0 +1,49 @@
+package org.bukkit.event;
+
+import org.bukkit.TestServer;
+import org.bukkit.plugin.PluginLoader;
+import org.bukkit.plugin.SimplePluginManager;
+import org.bukkit.plugin.TestPlugin;
+import org.bukkit.plugin.java.JavaPluginLoader;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class SyntheticEventTest {
+    @SuppressWarnings("deprecation")
+    @Test
+    public void test() {
+        final JavaPluginLoader loader = new JavaPluginLoader(TestServer.getInstance());
+        TestPlugin plugin = new TestPlugin(getClass().getName()) {
+            @Override
+            public PluginLoader getPluginLoader() {
+                return loader;
+            }
+        };
+        SimplePluginManager pluginManager = new SimplePluginManager(TestServer.getInstance(), null);
+
+        TestEvent event = new TestEvent(false);
+        Impl impl = new Impl();
+
+        pluginManager.registerEvents(impl, plugin);
+        pluginManager.callEvent(event);
+
+        Assert.assertEquals(1, impl.callCount);
+    }
+
+    public static abstract class Base<E extends Event> implements Listener {
+        int callCount = 0;
+
+        public void accept(E evt) {
+            System.out.println("Invk " + evt);
+            callCount++;
+        }
+    }
+
+    public static class Impl extends Base<TestEvent> {
+        @Override
+        @EventHandler
+        public void accept(TestEvent evt) {
+            super.accept(evt);
+        }
+    }
+}