Antimultirun - See https://facepunch.com/showthread.php?t=1547100

gamemode/config/config.lua

@@ -33,6 +33,8 @@ GM.Config.allowvehicleowning            = true
 GM.Config.allowvnocollide               = false
 -- alltalk - Enable for global chat, disable for local chat.
 GM.Config.alltalk                       = false
+-- antimultirun - Disallow people joining your server(s) twice on the same account
+GM.Config.antimultirun                  = true
 -- autovehiclelock - Enable/Disable automatic locking of a vehicle when a player exits it.
 GM.Config.autovehiclelock               = false
 -- babygod - people spawn godded (prevent spawn killing).

gamemode/modules/workarounds/sv_antimultirun.lua

@@ -0,0 +1,70 @@
+local kickMessage = [[You cannot join these server(s) twice with the same account.
+If you're a developer, please disable antimultirun in the DarkRP config.
+]]
+
+local function clearServerEntries()
+    MySQLite.query(string.format([[
+        DELETE FROM darkrp_serverplayer WHERE serverid = %s
+    ]], MySQLite.SQLStr(DarkRP.serverId)))
+end
+
+local function insertUid(uid)
+    MySQLite.query(string.format([[
+        INSERT INTO darkrp_serverplayer VALUES(%s, %s)
+    ]], uid, MySQLite.SQLStr(DarkRP.serverId)))
+end
+
+local function insertPlayer(ply)
+    insertUid(ply:SteamID64())
+end
+
+
+local function removePlayer(ply)
+    MySQLite.query(string.format([[
+        DELETE FROM darkrp_serverplayer WHERE uid = %s AND serverid = %s
+    ]], ply:SteamID64(), MySQLite.SQLStr(DarkRP.serverId)))
+end
+
+local function addHooks()
+    hook.Add("PlayerAuthed", "DarkRP_antimultirun", function(ply, steamId)
+        local uid = util.SteamIDTo64(steamId)
+        local userid = ply:UserID()
+
+        MySQLite.queryValue(string.format([[
+            SELECT serverid FROM darkrp_serverplayer WHERE serverid = %s AND uid = %s
+        ]], MySQLite.SQLStr(DarkRP.serverId), uid), function(sid)
+            if sid then
+                game.KickID(userid, kickMessage)
+            else
+                insertUid(uid)
+            end
+        end, error)
+    end)
+
+    hook.Add("PlayerDisconnected", "DarkRP_antimultirun", removePlayer)
+    hook.Add("ShutDown", "DarkRP_antimultirun", clearServerEntries)
+end
+
+hook.Add("DarkRPDBInitialized", "DarkRP_antimultirun", function()
+    if not GAMEMODE.Config.antimultirun then return end
+    if not MySQLite.isMySQL() then return end
+    if not game.IsDedicated() then return end
+
+    DarkRP.serverId = game.GetIPAddress()
+
+    MySQLite.query([[
+        CREATE TABLE IF NOT EXISTS darkrp_serverplayer(
+            uid BIGINT NOT NULL,
+            serverid VARCHAR(32) NOT NULL,
+            PRIMARY KEY(uid, serverid)
+        );
+    ]])
+
+    -- Clear this server's entries in case the server wasn't cleanly shut down
+    clearServerEntries()
+
+    -- Re-insert players currently in the game
+    fn.Map(insertPlayer, player.GetAll())
+
+    addHooks()
+end)