diff --git a/web/src/stores/conversation.ts b/web/src/stores/conversation.ts
index eeaf73f..1192ce6 100644
--- a/web/src/stores/conversation.ts
+++ b/web/src/stores/conversation.ts
@@ -2,6 +2,13 @@ import { create } from "zustand";
 import { api } from "../lib/api.ts";
 import { type Reaction } from "./message.ts";
 
+function parseDate(iso: string): number {
+  if (!iso) return 0;
+  const normalized = iso.includes("T") ? iso : iso.replace(" ", "T");
+  const t = new Date(normalized).getTime();
+  return isNaN(t) ? 0 : t;
+}
+
 export interface ConversationMember {
   id: string;
   username: string;
@@ -89,17 +96,26 @@ export const useConversationStore = create<ConversationState>((set, get) => ({
         `/conversations/${conversationId}/messages${params}`,
       );
       const list = Array.isArray(messages) ? messages : [];
-      set((state) => ({
-        messagesByConversation: {
-          ...state.messagesByConversation,
-          [conversationId]: list,
-        },
-        hasMoreByConversation: {
-          ...state.hasMoreByConversation,
-          [conversationId]: list.length >= 50,
-        },
-        isLoading: false,
-      }));
+      set((state) => {
+        const existing = state.messagesByConversation[conversationId] || [];
+        const map = new Map<string, ConversationMessage>();
+        existing.forEach((m) => map.set(m.id, m));
+        list.forEach((m) => map.set(m.id, m));
+        const merged = Array.from(map.values()).sort(
+          (a, b) => parseDate(a.created_at) - parseDate(b.created_at)
+        );
+        return {
+          messagesByConversation: {
+            ...state.messagesByConversation,
+            [conversationId]: merged,
+          },
+          hasMoreByConversation: {
+            ...state.hasMoreByConversation,
+            [conversationId]: list.length >= 50,
+          },
+          isLoading: false,
+        };
+      });
     } catch (error) {
       set({ isLoading: false, error: error instanceof Error ? error.message : "Failed" });
     }
@@ -118,17 +134,24 @@ export const useConversationStore = create<ConversationState>((set, get) => ({
         `/conversations/${conversationId}/messages?before=${encodeURIComponent(oldestId)}`,
       );
       const list = Array.isArray(older) ? older : [];
-      set((state) => ({
-        messagesByConversation: {
-          ...state.messagesByConversation,
-          [conversationId]: [...list, ...existing],
-        },
-        hasMoreByConversation: {
-          ...state.hasMoreByConversation,
-          [conversationId]: list.length >= 50,
-        },
-        isLoadingOlder: false,
-      }));
+      set((state) => {
+        const map = new Map<string, ConversationMessage>();
+        [...list, ...existing].forEach((m) => map.set(m.id, m));
+        const merged = Array.from(map.values()).sort(
+          (a, b) => parseDate(a.created_at) - parseDate(b.created_at)
+        );
+        return {
+          messagesByConversation: {
+            ...state.messagesByConversation,
+            [conversationId]: merged,
+          },
+          hasMoreByConversation: {
+            ...state.hasMoreByConversation,
+            [conversationId]: list.length >= 50,
+          },
+          isLoadingOlder: false,
+        };
+      });
     } catch {
       set({ isLoadingOlder: false });
     }
@@ -156,7 +179,7 @@ export const useConversationStore = create<ConversationState>((set, get) => ({
         messagesByConversation: {
           ...state.messagesByConversation,
           [message.conversation_id]: [...existing, message]
-            .sort((a, b) => new Date(a.created_at).getTime() - new Date(b.created_at).getTime()),
+            .sort((a, b) => parseDate(a.created_at) - parseDate(b.created_at)),
         },
       };
     });