Disallow duplicate eth addresses

Tools/BiblioTech/Commands/UserAssociateCommand.cs

@@ -27,8 +27,17 @@ namespace BiblioTech.Commands
                 return;
             }
 
-            Program.UserRepo.AssociateUserWithAddress(userId, data);
+            // private commands
+
+            var result = Program.UserRepo.AssociateUserWithAddress(userId, data);
+            if (result)
+            {
                 await context.Command.FollowupAsync("Done! Thank you for joining the test net!");
             }
+            else
+            {
+                await context.Command.FollowupAsync("That didn't work.");
+            }
+        }
     }
 }

Tools/BiblioTech/Options/EthAddressOption.cs

@@ -29,8 +29,8 @@ namespace BiblioTech.Options
             }
 
             if (!AddressUtil.Current.IsValidAddressLength(ethAddressStr) ||
-                !AddressUtil.Current.IsValidEthereumAddressHexFormat(ethAddressStr) ||
+                !AddressUtil.Current.IsValidEthereumAddressHexFormat(ethAddressStr))
-                !AddressUtil.Current.IsChecksumAddress(ethAddressStr))
+                // !AddressUtil.Current.IsChecksumAddress(ethAddressStr)) - this might make a good option later, but for now it might just annoy users.
             {
                 await context.Command.FollowupAsync("EthAddress is not valid.");
                 return null;

Tools/BiblioTech/UserRepo.cs

@@ -8,11 +8,11 @@ namespace BiblioTech
     {
         private readonly object repoLock = new object();
 
-        public void AssociateUserWithAddress(ulong discordId, EthAddress address)
+        public bool AssociateUserWithAddress(ulong discordId, EthAddress address)
         {
             lock (repoLock)
             {
-                SetUserAddress(discordId, address);
+                return SetUserAddress(discordId, address);
             }
         }
 
@@ -79,12 +79,18 @@ namespace BiblioTech
             return result.ToArray();
         }
 
-        private void SetUserAddress(ulong discordId, EthAddress? address)
+        private bool SetUserAddress(ulong discordId, EthAddress? address)
         {
+            if (IsAddressUsed(address))
+            {
+                return false;
+            }
+
             var user = GetOrCreate(discordId);
             user.CurrentAddress = address;
             user.AssociateEvents.Add(new UserAssociateAddressEvent(DateTime.UtcNow, address));
             SaveUser(user);
+            return true;
         }
 
         private User GetOrCreate(ulong discordId)
@@ -104,6 +110,29 @@ namespace BiblioTech
             return newUser;
         }
 
+        private bool IsAddressUsed(EthAddress? address)
+        {
+            if (address == null) return false;
+
+            // If this becomes a performance problem, switch to in-memory cached list.
+            var files = Directory.GetFiles(Program.Config.UserDataPath);
+            foreach (var file in files)
+            {
+                try
+                {
+                    var user = JsonConvert.DeserializeObject<User>(File.ReadAllText(file))!;
+                    if (user.CurrentAddress != null &&
+                        user.CurrentAddress.Address == address.Address)
+                    {
+                        return true;
+                    }
+                }
+                catch { }
+            }
+
+            return false;
+        }
+
         private void SaveUser(User user)
         {
             var filename = GetFilename(user.DiscordId);