diff --git a/src/bot_turns.rs b/src/bot_turns.rs
index c69971f8..130fa717 100644
--- a/src/bot_turns.rs
+++ b/src/bot_turns.rs
@@ -13,6 +13,11 @@ use std::collections::HashMap;
 /// between human resets.
 pub const HARD_BOT_TURN_LIMIT: u32 = 1000;
 
+/// Stable prefix used in all bot turn limit warning messages.
+/// Referenced by the dedup check in the Discord adapter — changing this
+/// string requires updating the dedup check too.
+pub const BOT_TURN_LIMIT_WARNING_PREFIX: &str = "⚠️ Bot turn limit reached";
+
 #[derive(Debug, PartialEq, Eq)]
 pub enum TurnResult {
     /// Counter below limits — continue normally.
@@ -75,8 +80,9 @@ impl BotTurnTracker {
                 severity: TurnSeverity::Soft,
                 turns: n,
                 user_message: format!(
-                    "⚠️ Bot turn limit reached ({n}/{soft}). \
+                    "{} ({n}/{soft}). \
                      A human must reply in this thread to continue bot-to-bot conversation.",
+                    BOT_TURN_LIMIT_WARNING_PREFIX,
                     soft = self.soft_limit,
                 ),
             },
@@ -276,9 +282,11 @@ mod tests {
             TurnAction::WarnAndStop {
                 severity: TurnSeverity::Soft,
                 turns: 3,
-                user_message: "⚠️ Bot turn limit reached (3/3). \
-                               A human must reply in this thread to continue bot-to-bot conversation."
-                    .to_string(),
+                user_message: format!(
+                    "{} (3/3). \
+                     A human must reply in this thread to continue bot-to-bot conversation.",
+                    BOT_TURN_LIMIT_WARNING_PREFIX,
+                ),
             },
         );
     }
diff --git a/src/discord.rs b/src/discord.rs
index 837e8048..6355e17d 100644
--- a/src/discord.rs
+++ b/src/discord.rs
@@ -1,7 +1,7 @@
 use crate::acp::protocol::ConfigOption;
 use crate::acp::ContentBlock;
 use crate::adapter::{AdapterRouter, ChannelRef, ChatAdapter, MessageRef, SenderContext};
-use crate::bot_turns::{BotTurnTracker, TurnAction, TurnSeverity};
+use crate::bot_turns::{BotTurnTracker, TurnAction, TurnSeverity, BOT_TURN_LIMIT_WARNING_PREFIX};
 use crate::config::{AllowBots, AllowUsers, SttConfig};
 use crate::format;
 use crate::media;
@@ -397,7 +397,25 @@ impl EventHandler for Handler {
                                 .bot_participated_in_thread(&ctx.http, msg.channel_id, bot_id)
                                 .await;
                             if participated {
-                                let _ = msg.channel_id.say(&ctx.http, &user_message).await;
+                                // Dedup: skip if another bot already posted the same
+                                // warning in this thread. Prevents N duplicate warnings
+                                // when N bot processes each hit the soft limit. (#530)
+                                let recent = msg
+                                    .channel_id
+                                    .messages(
+                                        &ctx.http,
+                                        serenity::builder::GetMessages::new().limit(10),
+                                    )
+                                    .await
+                                    .unwrap_or_default();
+                                let pairs: Vec<(bool, &str)> = recent
+                                    .iter()
+                                    .map(|m| (m.author.bot, m.content.as_str()))
+                                    .collect();
+                                let already_warned = turn_limit_warning_present(&pairs);
+                                if !already_warned {
+                                    let _ = msg.channel_id.say(&ctx.http, &user_message).await;
+                                }
                             }
                         }
                         return;
@@ -2151,10 +2169,26 @@ fn should_process_user_message(
     }
 }
 
+/// Returns true if any bot message in `messages` contains a turn limit warning.
+/// Used to dedup `WarnAndStop` across multiple bot processes sharing a thread. (#530)
+/// Note: this is best-effort — a narrow race window exists where two bots fetch
+/// simultaneously and both see no warning, resulting in a duplicate. For most
+/// deployments this is acceptable; strict once-only semantics would require
+/// shared state (e.g. gateway-owned emission or distributed lock).
+///
+/// Accepts `(is_bot, content)` pairs so the logic can be unit-tested without
+/// constructing `serenity::model::channel::Message` values (see existing test
+/// boundary comment at `format_thread_export`).
+fn turn_limit_warning_present(messages: &[(bool, &str)]) -> bool {
+    messages
+        .iter()
+        .any(|(is_bot, content)| *is_bot && content.contains(BOT_TURN_LIMIT_WARNING_PREFIX))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::bot_turns::{TurnResult, HARD_BOT_TURN_LIMIT};
+    use crate::bot_turns::{TurnResult, HARD_BOT_TURN_LIMIT, BOT_TURN_LIMIT_WARNING_PREFIX};
 
     // --- resolve_mentions tests ---
 
@@ -2939,4 +2973,28 @@ mod tests {
     fn normal_channel_creates_thread() {
         assert!(!should_skip_thread_creation(false, false));
     }
+
+    // --- WarnAndStop dedup tests (#530) ---
+
+    #[test]
+    fn dedup_detects_existing_bot_warning() {
+        let msg = format!("{} (20/20). A human must reply.", BOT_TURN_LIMIT_WARNING_PREFIX);
+        assert!(turn_limit_warning_present(&[(true, &msg)]));
+    }
+
+    #[test]
+    fn dedup_ignores_human_warning_text() {
+        let msg = format!("{} (20/20). A human must reply.", BOT_TURN_LIMIT_WARNING_PREFIX);
+        assert!(!turn_limit_warning_present(&[(false, &msg)]));
+    }
+
+    #[test]
+    fn dedup_returns_false_when_no_warning() {
+        assert!(!turn_limit_warning_present(&[(true, "hello"), (false, "world")]));
+    }
+
+    #[test]
+    fn dedup_returns_false_for_empty_messages() {
+        assert!(!turn_limit_warning_present(&[]));
+    }
 }