diff --git a/.changeset/fix-mention-offset-user-change.md b/.changeset/fix-mention-offset-user-change.md
new file mode 100644
index 00000000..04315ca9
--- /dev/null
+++ b/.changeset/fix-mention-offset-user-change.md
@@ -0,0 +1,5 @@
+---
+"@chat-adapter/slack": patch
+---
+
+Fix duplicate mention resolution by using the replace callback offset instead of indexOf. Invalidate user cache on Slack user_change events so display name updates are picked up immediately.
diff --git a/packages/adapter-slack/src/index.test.ts b/packages/adapter-slack/src/index.test.ts
index c4e743d9..91cd0989 100644
--- a/packages/adapter-slack/src/index.test.ts
+++ b/packages/adapter-slack/src/index.test.ts
@@ -4927,7 +4927,7 @@ describe("reverse user lookup", () => {
         }
       ).parseSlackMessage(event, threadId);
 
-      // Wait for fire-and-forget to complete
+      // Allow participant tracking to complete
       await new Promise((resolve) => setTimeout(resolve, 10));
 
       const participants = await state.getList(
@@ -4936,4 +4936,49 @@ describe("reverse user lookup", () => {
       expect(participants).toContain("U_SENDER_1");
     });
   });
+
+  describe("user_change event", () => {
+    it("invalidates user cache on profile change", async () => {
+      const state = createMockState();
+      const adapter = createSlackAdapter({
+        botToken: "xoxb-test-token",
+        signingSecret: secret,
+        logger: mockLogger,
+      });
+      await adapter.initialize(createMockChatInstance(state));
+
+      // Seed user cache
+      await state.set(
+        "slack:user:U_DOM_123",
+        { displayName: "dominik", realName: "Dominik G" },
+        8 * 24 * 60 * 60 * 1000
+      );
+
+      const body = JSON.stringify({
+        type: "event_callback",
+        event: {
+          type: "user_change",
+          event_ts: "1234567890.123456",
+          user: {
+            id: "U_DOM_123",
+            name: "dominik",
+            real_name: "Dominik New",
+            profile: {
+              display_name: "dom_new",
+              real_name: "Dominik New",
+            },
+          },
+        },
+      });
+      const request = createWebhookRequest(body, secret);
+      const response = await adapter.handleWebhook(request);
+
+      expect(response.status).toBe(200);
+
+      await new Promise((resolve) => setTimeout(resolve, 50));
+
+      const cached = await state.get("slack:user:U_DOM_123");
+      expect(cached).toBeNull();
+    });
+  });
 });
diff --git a/packages/adapter-slack/src/index.ts b/packages/adapter-slack/src/index.ts
index dfc16aa1..ebc419d4 100644
--- a/packages/adapter-slack/src/index.ts
+++ b/packages/adapter-slack/src/index.ts
@@ -244,6 +244,18 @@ interface SlackMemberJoinedChannelEvent {
   user: string;
 }
 
+/** Slack user_change event payload */
+interface SlackUserChangeEvent {
+  event_ts: string;
+  type: "user_change";
+  user: {
+    id: string;
+    name?: string;
+    real_name?: string;
+    profile?: { display_name?: string; real_name?: string };
+  };
+}
+
 /** Slack webhook payload envelope */
 interface SlackWebhookPayload {
   challenge?: string;
@@ -253,7 +265,8 @@ interface SlackWebhookPayload {
     | SlackAssistantThreadStartedEvent
     | SlackAssistantContextChangedEvent
     | SlackAppHomeOpenedEvent
-    | SlackMemberJoinedChannelEvent;
+    | SlackMemberJoinedChannelEvent
+    | SlackUserChangeEvent;
   event_id?: string;
   event_time?: number;
   team_id?: string;
@@ -916,6 +929,8 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
           event as SlackMemberJoinedChannelEvent,
           options
         );
+      } else if (event.type === "user_change") {
+        this.handleUserChange(event as SlackUserChangeEvent);
       }
     }
   }
@@ -1573,6 +1588,21 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
     );
   }
 
+  private async handleUserChange(event: SlackUserChangeEvent): Promise<void> {
+    if (!this.chat) {
+      return;
+    }
+
+    try {
+      await this.chat.getState().delete(`slack:user:${event.user.id}`);
+    } catch (error) {
+      this.logger.warn("Failed to invalidate user cache", {
+        userId: event.user.id,
+        error,
+      });
+    }
+  }
+
   /**
    * Publish a Home tab view for a user.
    * Slack API: views.publish
@@ -2021,32 +2051,34 @@ export class SlackAdapter implements Adapter<SlackThreadId, unknown> {
     }
 
     // Replace mentions in text
-    return text.replace(mentionPattern, (match, name: string) => {
-      const idx = text.indexOf(match);
-      if (idx > 0 && text[idx - 1] === "<") {
-        return match;
-      }
-      if (SLACK_USER_ID_EXACT_PATTERN.test(name)) {
-        return match;
-      }
+    return text.replace(
+      mentionPattern,
+      (match, name: string, offset: number) => {
+        if (offset > 0 && text[offset - 1] === "<") {
+          return match;
+        }
+        if (SLACK_USER_ID_EXACT_PATTERN.test(name)) {
+          return match;
+        }
 
-      const userIds = mentions.get(name.toLowerCase());
-      if (!userIds || userIds.length === 0) {
-        return match;
-      }
-      if (userIds.length === 1) {
-        return `<@${userIds[0]}>`;
-      }
-      // Disambiguate using thread participants
-      if (participants) {
-        const inThread = userIds.filter((id) => participants.has(id));
-        if (inThread.length === 1) {
-          return `<@${inThread[0]}>`;
+        const userIds = mentions.get(name.toLowerCase());
+        if (!userIds || userIds.length === 0) {
+          return match;
+        }
+        if (userIds.length === 1) {
+          return `<@${userIds[0]}>`;
+        }
+        // Disambiguate using thread participants
+        if (participants) {
+          const inThread = userIds.filter((id) => participants.has(id));
+          if (inThread.length === 1) {
+            return `<@${inThread[0]}>`;
+          }
         }
+        // Still ambiguous — leave as plain text
+        return match;
       }
-      // Still ambiguous — leave as plain text
-      return match;
-    });
+    );
   }
 
   /**