Avoid duplicate logins.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -118,9 +118,15 @@ public class VelocityServer implements ProxyServer {
         return httpClient;
     }
 
-    public void registerConnection(ConnectedPlayer connection) {
-        connectionsByName.put(connection.getUsername(), connection);
-        connectionsByUuid.put(connection.getUniqueId(), connection);
+    public boolean registerConnection(ConnectedPlayer connection) {
+        if (connectionsByName.putIfAbsent(connection.getUsername(), connection) != null) {
+            return false;
+        }
+        if (connectionsByUuid.putIfAbsent(connection.getUniqueId(), connection) != null) {
+            connectionsByName.remove(connection.getUsername(), connection);
+            return false;
+        }
+        return true;
     }
 
     public void unregisterConnection(ConnectedPlayer connection) {

proxy/src/main/java/com/velocitypowered/proxy/connection/client/LoginSessionHandler.java

@@ -136,11 +136,15 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         success.setUuid(profile.idAsUuid());
         inbound.write(success);
 
-        logger.info("{} has connected", player);
         inbound.setAssociation(player);
         inbound.setState(StateRegistry.PLAY);
+
+        if (!VelocityServer.getServer().registerConnection(player)) {
+            inbound.closeWith(Disconnect.create(TextComponent.of("You are already on this proxy!", TextColor.RED)));
+        }
+
+        logger.info("{} has connected", player);
         inbound.setSessionHandler(new InitialConnectSessionHandler(player));
-        VelocityServer.getServer().registerConnection(player);
         player.createConnectionRequest(toTry.get()).fireAndForget();
     }
 }