fix(trading): prevent duplicate active trades

Guard RoomTradeManager.startTrade while holding the activeTrades lock so concurrent trade starts cannot register the same participant in multiple active trades before room status updates settle.

Add a contract test covering the lock-scoped participant guard and keep the existing trade safety tests green.

Emulator/src/main/java/com/eu/habbo/habbohotel/rooms/RoomTradeManager.java

@@ -19,8 +19,13 @@ public class RoomTradeManager {
      * Starts a trade between two users.
      */
     public void startTrade(Habbo userOne, Habbo userTwo) {
-        RoomTrade trade = new RoomTrade(userOne, userTwo, this.room);
+        RoomTrade trade;
         synchronized (this.activeTrades) {
+            if (this.hasActiveTrade(userOne) || this.hasActiveTrade(userTwo)) {
+                return;
+            }
+
+            trade = new RoomTrade(userOne, userTwo, this.room);
             this.activeTrades.add(trade);
         }
 
@@ -58,4 +63,16 @@ public class RoomTradeManager {
     public THashSet<RoomTrade> getActiveTrades() {
         return this.activeTrades;
     }
+
+    private boolean hasActiveTrade(Habbo user) {
+        for (RoomTrade trade : this.activeTrades) {
+            for (RoomTradeUser habbo : trade.getRoomTradeUsers()) {
+                if (habbo.getHabbo() == user) {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
 }

Emulator/src/test/java/com/eu/habbo/habbohotel/rooms/RoomTradeManagerContractTest.java

@@ -0,0 +1,30 @@
+package com.eu.habbo.habbohotel.rooms;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import org.junit.jupiter.api.Test;
+
+class RoomTradeManagerContractTest {
+    private static String roomTradeManagerSource() throws Exception {
+        return Files.readString(Path.of("src/main/java/com/eu/habbo/habbohotel/rooms/RoomTradeManager.java"));
+    }
+
+    @Test
+    void startTradeRejectsParticipantsAlreadyInActiveTradeInsideLock() throws Exception {
+        String source = roomTradeManagerSource();
+        int synchronizedBlock = source.indexOf("synchronized (this.activeTrades)");
+        int activeGuard = source.indexOf("hasActiveTrade(userOne) || this.hasActiveTrade(userTwo)");
+        int addTrade = source.indexOf("this.activeTrades.add(trade)");
+
+        assertTrue(synchronizedBlock > -1, "RoomTradeManager.startTrade must lock activeTrades before mutation");
+        assertTrue(activeGuard > synchronizedBlock,
+                "startTrade must check both participants for an existing active trade while holding the activeTrades lock");
+        assertTrue(activeGuard < addTrade,
+                "duplicate participant guard must run before a new RoomTrade is added");
+        assertTrue(source.contains("private boolean hasActiveTrade(Habbo user)"),
+                "active trade lookup should be reusable under the same activeTrades lock");
+    }
+}