diff --git a/server/gameconnection.py b/server/gameconnection.py
index 2f3d9f6b1..73774be53 100644
--- a/server/gameconnection.py
+++ b/server/gameconnection.py
@@ -114,6 +114,13 @@ async def _handle_idle_state(self):
         """
         assert self.game
 
+        if self.state is not GameConnectionState.INITIALIZING:
+            self._logger.warning(
+                "Ignoring unexpected 'Idle' state when state was %s",
+                self.state,
+            )
+            return
+
         if self.player == self.game.host:
             self.game.state = GameState.LOBBY
             self._state = GameConnectionState.CONNECTED_TO_HOST
@@ -132,6 +139,13 @@ async def _handle_lobby_state(self):
         """
         player_state = self.player.state
         if player_state == PlayerState.HOSTING:
+            if self.game.hosted_at is not None:
+                self._logger.warning(
+                    "Ignoring unexpected 'Lobby' state sent when the game was "
+                    "already hosted.",
+                )
+                return
+
             await self.send_HostGame(self.game.map.folder_name)
             self.game.set_hosted()
         # If the player is joining, we connect him to host
diff --git a/tests/integration_tests/test_game.py b/tests/integration_tests/test_game.py
index 3c90ecb45..62dfb51e7 100644
--- a/tests/integration_tests/test_game.py
+++ b/tests/integration_tests/test_game.py
@@ -1320,6 +1320,48 @@ async def test_ladder_game_not_joinable(lobby_server):
     }
 
 
+@fast_forward(60)
+async def test_gamestate_lobby_double_game_instance(lobby_server):
+    """There was a bug in the client that was causing multiple game instances
+    to launch simultaneously. This could cause the GpgNet messages in the game
+    startup sequence to be sent twice.
+    """
+    _, _, host_proto = await connect_and_sign_in(
+        ("test", "test_password"), lobby_server
+    )
+    _, _, guest_proto = await connect_and_sign_in(
+        ("Rhiza", "puff_the_magic_dragon"), lobby_server
+    )
+    await read_until_command(host_proto, "game_info")
+    await read_until_command(guest_proto, "game_info")
+
+    await host_proto.send_message({
+        "command": "game_host",
+        "mod": "faf",
+        "visibility": "public",
+    })
+    msg = await read_until_command(host_proto, "game_launch")
+    game_id = int(msg["uid"])
+
+    for _ in range(2):
+        await host_proto.send_message({
+            "target": "game",
+            "command": "GameState",
+            "args": ["Idle"]
+        })
+
+    for _ in range(2):
+        await host_proto.send_message({
+            "target": "game",
+            "command": "GameState",
+            "args": ["Lobby"]
+        })
+
+    await read_until_command(host_proto, "HostGame", target="game")
+
+    await join_game(guest_proto, game_id)
+
+
 @pytest.mark.flaky
 @fast_forward(60)
 async def test_gamestate_ended_clears_references(
diff --git a/tests/unit_tests/test_gameconnection.py b/tests/unit_tests/test_gameconnection.py
index f39b9facb..06821e838 100644
--- a/tests/unit_tests/test_gameconnection.py
+++ b/tests/unit_tests/test_gameconnection.py
@@ -99,18 +99,33 @@ async def test_connect_to_peer_disconnected(game_connection):
     await game_connection.connect_to_peer(peer)
 
 
-async def test_handle_action_GameState_idle_adds_connection(
+async def test_handle_action_GameState_idle_hosting(
     game: Game,
     game_connection: GameConnection,
     players
 ):
-    players.joining.game = game
     game_connection.player = players.hosting
     game_connection.game = game
 
     await game_connection.handle_action("GameState", ["Idle"])
 
-    game.add_game_connection.assert_called_with(game_connection)
+    game.add_game_connection.assert_called_once_with(game_connection)
+
+
+async def test_handle_action_GameState_idle_joining(
+    game: Game,
+    game_connection: GameConnection,
+    players
+):
+    game_connection.player = players.joining
+    game_connection.game = game
+
+    await game_connection.handle_action("GameState", ["Idle"])
+
+    assert players.joining.state == PlayerState.JOINING
+    assert game_connection.state == GameConnectionState.INITIALIZED
+
+    game.add_game_connection.assert_not_called()
 
 
 async def test_handle_action_GameState_idle_sets_player_state(