diff --git a/frontends/slack/src/main/kotlin/com/gatehill/corebot/chat/SlackChatServiceImpl.kt b/frontends/slack/src/main/kotlin/com/gatehill/corebot/chat/SlackChatServiceImpl.kt
index 1bd2be5b..4302d245 100644
--- a/frontends/slack/src/main/kotlin/com/gatehill/corebot/chat/SlackChatServiceImpl.kt
+++ b/frontends/slack/src/main/kotlin/com/gatehill/corebot/chat/SlackChatServiceImpl.kt
@@ -2,10 +2,13 @@ package com.gatehill.corebot.chat
 
 import com.gatehill.corebot.action.model.TriggerContext
 import com.gatehill.corebot.config.ChatSettings
+import com.google.common.cache.CacheBuilder
 import com.ullink.slack.simpleslackapi.SlackSession
+import com.ullink.slack.simpleslackapi.events.SlackMessagePosted
 import com.ullink.slack.simpleslackapi.listeners.SlackMessagePostedListener
 import org.apache.logging.log4j.LogManager
 import org.apache.logging.log4j.Logger
+import java.util.concurrent.TimeUnit
 import java.util.regex.Pattern
 import javax.inject.Inject
 
@@ -20,6 +23,14 @@ open class SlackChatServiceImpl @Inject constructor(private val sessionService:
     private val logger: Logger = LogManager.getLogger(SlackChatServiceImpl::class.java)
     private val messageMatcher = Pattern.compile("<@(?<botUser>[a-zA-Z0-9]+)>:?(\\s(?<command>.+))?")
 
+    /**
+     * Caches message IDs for de-duplication purposes.
+     * This is a work-around for a Slack library bug: [https://github.com/Ullink/simple-slack-api/issues/180]
+     */
+    private val messageIdCache = CacheBuilder.newBuilder()
+            .expireAfterAccess(ChatSettings.chat.messageIdCache, TimeUnit.SECONDS)
+            .build<String, String>()
+
     override fun listenForEvents() {
         messagePostedListeners.forEach { sessionService.session.addMessagePostedListener(it) }
     }
@@ -40,6 +51,9 @@ open class SlackChatServiceImpl @Inject constructor(private val sessionService:
         // ignore own messages
         if (session.sessionPersona().id == event.sender.id) return@SlackMessagePostedListener
 
+        // filter duplicates
+        if (isDuplicate(event, session)) return@SlackMessagePostedListener
+
         val trigger = TriggerContext(event.channel.id, event.sender.id, event.sender.userName, event.timestamp, event.threadTimestamp)
 
         try {
@@ -64,6 +78,26 @@ open class SlackChatServiceImpl @Inject constructor(private val sessionService:
         }
     })
 
+    /**
+     * Determine whether this message has been processed already.
+     */
+    private fun isDuplicate(event: SlackMessagePosted, session: SlackSession): Boolean {
+        // According to https://api.slack.com/events-api
+        // 'The combination of event_ts, team_id, user_id, or channel_id is
+        // intended to be unique. This field is included with every inner event type'.
+        val uniqueId = "${event.timeStamp}_${session.team.id}_${event.sender?.id}_${event.channel?.id}"
+
+        synchronized(messageIdCache) {
+            val found = (uniqueId == messageIdCache.getIfPresent(uniqueId))
+            if (found) {
+                logger.debug("Duplicate message received: $event")
+            } else {
+                messageIdCache.put(uniqueId, uniqueId)
+            }
+            return found
+        }
+    }
+
     /***
      * Execute the `block` if the message is addressed to the bot and return its result.
      */
diff --git a/frontends/slack/src/main/kotlin/com/gatehill/corebot/config/ChatSettings.kt b/frontends/slack/src/main/kotlin/com/gatehill/corebot/config/ChatSettings.kt
index b5cac235..815a171c 100644
--- a/frontends/slack/src/main/kotlin/com/gatehill/corebot/config/ChatSettings.kt
+++ b/frontends/slack/src/main/kotlin/com/gatehill/corebot/config/ChatSettings.kt
@@ -12,6 +12,11 @@ object ChatSettings {
             (System.getenv("SLACK_CHANNELS") ?: "corebot").split(",").map(String::trim)
         }
         val postJoinMessage by lazy { System.getenv("SLACK_ENABLE_JOIN_MESSAGE")?.toBoolean() ?: true }
+
+        /**
+         * The time in seconds to cache message IDs for de-duplication purposes.
+         */
+        val messageIdCache by lazy { System.getenv("SLACK_MESSAGE_ID_CACHE")?.toLong() ?: 120 }
     }
 
     class Threads {