Add the project

.gitignore

@@ -10,3 +10,9 @@ buildNumber.properties
 
 # Avoid ignoring Maven wrapper jar file (.jar files are usually ignored)
 !/.mvn/wrapper/maven-wrapper.jar
+
+# IDE
+/.settings/
+/lib/
+/.classpath
+/.project

pom.xml

@@ -0,0 +1,19 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <groupId>com.vanillage.onlinecheck</groupId>
+  <artifactId>onlinecheck</artifactId>
+  <version>1.0</version>
+  <name>OnlineCheck</name>
+  <dependencies>
+    <dependency>
+      <groupId>com.vanillage.minecraftalphaserver</groupId>
+      <artifactId>minecraftalphaserver</artifactId>
+      <version>1.0</version>
+      <scope>system</scope>
+      <systemPath>${project.basedir}/lib/minecraft_server_unobfuscated.jar</systemPath>
+    </dependency>
+  </dependencies>
+  <build>
+    <finalName>OnlineCheck</finalName>
+  </build>
+</project>

src/main/java/com/vanillage/onlinecheck/OnlineCheck.java

@@ -0,0 +1,17 @@
+package com.vanillage.onlinecheck;
+
+import com.vanillage.minecraftalphaserver.event.PlayerLoggedInListener;
+import com.vanillage.minecraftalphaserver.plugin.Plugin;
+
+public final class OnlineCheck extends Plugin {
+    @Override
+    public void onEnable() {
+        getMinecraftServer().getPluginManager().getEventManager().registerListener((PlayerLoggedInListener) new OnlineCheckPlayerLoggedInListener(this));
+        getMinecraftServer().log(getName() + " enabled");
+    }
+
+    @Override
+    public void onDisable() {
+        getMinecraftServer().log(getName() + " disabled");
+    }
+}

src/main/java/com/vanillage/onlinecheck/OnlineCheckPlayerLoggedInListener.java

@@ -0,0 +1,61 @@
+package com.vanillage.onlinecheck;
+
+import com.vanillage.minecraftalphaserver.event.PlayerLoggedInEvent;
+import com.vanillage.minecraftalphaserver.event.PlayerLoggedInListener;
+
+public final class OnlineCheckPlayerLoggedInListener extends PlayerLoggedInListener {
+    private final OnlineCheck plugin;
+    private final int priority;
+    private String kickReason;
+
+    public OnlineCheckPlayerLoggedInListener(OnlineCheck plugin) {
+        this(plugin, -5, null);
+    }
+
+    public OnlineCheckPlayerLoggedInListener(OnlineCheck plugin, int priority) {
+        this(plugin, priority, null);
+    }
+
+    public OnlineCheckPlayerLoggedInListener(OnlineCheck plugin, String kickReason) {
+        this(plugin, -5, kickReason);
+    }
+
+    public OnlineCheckPlayerLoggedInListener(OnlineCheck plugin, int priority, String kickReason) {
+        if (plugin == null) {
+            throw new IllegalArgumentException("plugin cannot be null");
+        }
+
+        this.plugin = plugin;
+        this.priority = priority;
+        this.kickReason = kickReason == null ? "<username> is already online" : kickReason;
+    }
+
+    public OnlineCheck getPlugin() {
+        return plugin;
+    }
+
+    public String getKickReason() {
+        return kickReason;
+    }
+
+    public void setKickReason(String kickReason) {
+        if (kickReason == null) {
+            throw new IllegalArgumentException("kickReason cannot be null");
+        }
+
+        this.kickReason = kickReason;
+    }
+
+    @Override
+    public void onEvent(PlayerLoggedInEvent event) {
+        if (!event.isDenied() && plugin.getMinecraftServer().configManager.getPlayerEntity(event.getPacket().username) != null) {
+            event.deny();
+            event.setKickReason(kickReason.replace("<username>", event.getPacket().username));
+        }
+    }
+
+    @Override
+    public int getPriority() {
+        return priority;
+    }
+}