diff --git a/truthseeker/logic/game_logic.py b/truthseeker/logic/game_logic.py
index b4aeaf7..e0ed3a1 100644
--- a/truthseeker/logic/game_logic.py
+++ b/truthseeker/logic/game_logic.py
@@ -56,7 +56,14 @@ class Game:
         self.members.append(self.owner)
         return self.owner
 
+    def get_member(self, username):
+        for member in self.members:
+            if member.username == username:
+                return member
+
     def add_member(self, username):
+        if self.get_member(username):
+            return None
         member = Member(username)
         self.members.append(member)
         return member
diff --git a/truthseeker/routes/routes_api.py b/truthseeker/routes/routes_api.py
index def7d41..76180d4 100644
--- a/truthseeker/routes/routes_api.py
+++ b/truthseeker/routes/routes_api.py
@@ -39,8 +39,8 @@ def join_game():
     if game == None:
         return {"error": 1, "msg": "game does not exist"}
     
-
-    game.add_member(username)
+    if not game.add_member(username):
+        return {"error": 1, "msg": f"Username '{username}' already used in game {game.game_id}"}
 
     flask.session["game_id"] = game.game_id
     flask.session["is_owner"] = False