diff --git a/inventoryManager.lua b/inventoryManager.lua
index 0fec772..c5c7509 100644
--- a/inventoryManager.lua
+++ b/inventoryManager.lua
@@ -28,6 +28,34 @@ local BROADCAST_INTERVAL = 1   -- seconds between state broadcasts
 local CRAFT_CHANNEL       = 4203
 local CRAFT_REPLY_CHANNEL = 4204
 
+-------------------------------------------------
+-- Idempotent command tracking
+-------------------------------------------------
+
+local processedCmdIds = {}   -- commandId -> os.epoch("utc") ms
+local CMD_TTL_MS      = 300000  -- 5 minutes
+
+local function isCommandDuplicate(commandId)
+    if not commandId then return false end
+    local entry = processedCmdIds[commandId]
+    if entry and (os.epoch("utc") - entry) < CMD_TTL_MS then
+        return true
+    end
+    return false
+end
+
+local function recordCommandId(commandId)
+    if not commandId then return end
+    processedCmdIds[commandId] = os.epoch("utc")
+end
+
+local function cleanupCommandIds()
+    local cutoff = os.epoch("utc") - CMD_TTL_MS
+    for id, ts in pairs(processedCmdIds) do
+        if ts < cutoff then processedCmdIds[id] = nil end
+    end
+end
+
 -------------------------------------------------
 -- Load config from file if present
 -------------------------------------------------