diff --git a/SVSim.EmulatedEntrypoint/Controllers/GiftController.cs b/SVSim.EmulatedEntrypoint/Controllers/GiftController.cs
index 9d97a99..d0250c4 100644
--- a/SVSim.EmulatedEntrypoint/Controllers/GiftController.cs
+++ b/SVSim.EmulatedEntrypoint/Controllers/GiftController.cs
@@ -77,8 +77,14 @@ public class GiftController : SVSimController
         // the pattern in TutorialController.Update and to make the intent clear.
         // AsSplitQuery is the default-safe pattern when including viewer collections
         // (project memory: project_ef_split_query).
+        //
+        // ThenInclude(i => i.Item) is load-bearing: OwnedItemEntry.Item is a separate non-owned
+        // entity whose default initialiser is `new ItemEntry()` (Id=0). Without the explicit
+        // ThenInclude, RewardGrantService.ApplyAsync's `FirstOrDefault(i => i.Item.Id == ...)`
+        // never matches a pre-existing row → falls through to add a duplicate → (ViewerId, ItemId)
+        // unique index throws on SaveChanges (project_ef_nav_include_pitfall).
         var viewer = await _db.Viewers
-            .Include(v => v.Items)
+            .Include(v => v.Items).ThenInclude(i => i.Item)
             .Include(v => v.MissionData)
             .AsSplitQuery()
             .FirstAsync(v => v.Id == viewerId);
diff --git a/SVSim.UnitTests/Controllers/GiftControllerTests.cs b/SVSim.UnitTests/Controllers/GiftControllerTests.cs
index 1e9f543..8f779fa 100644
--- a/SVSim.UnitTests/Controllers/GiftControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/GiftControllerTests.cs
@@ -119,6 +119,45 @@ public class GiftControllerTests
         Assert.That(await factory.GetViewerTutorialStateAsync(viewerId), Is.EqualTo(41));
     }
 
+    [Test]
+    public async Task GiftReceive_with_pre_owned_item_increments_existing_row()
+    {
+        using var factory = new SVSimTestFactory();
+        await factory.SeedGlobalsAsync();
+        long viewerId = await factory.SeedViewerAsync(tutorialState: 31);
+
+        // Seed item 1 (= the 3-item gift's reward_detail_id) with count=5 pre-existing.
+        // Any non-tutorial source could leave a viewer here — battlepass, future reward,
+        // admin import. Gift 71478628 grants +3 of item 1; the existing row must be
+        // found and incremented, not duplicated. The (ViewerId, ItemId) unique index
+        // added 2026-05-25 would otherwise throw on SaveChanges → 500 to the client.
+        await factory.SeedOwnedItemAsync(viewerId, itemId: 1, count: 5, itemName: "PreOwnedItem");
+
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var json = $$"""{"present_id_array":["71478628"],"state":1,{{BaseAuthBlock}}}""";
+        var response = await client.PostAsync("/tutorial/gift_receive",
+            new StringContent(json, Encoding.UTF8, "application/json"));
+
+        var bodyStr = await response.Content.ReadAsStringAsync();
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), bodyStr);
+
+        // Existing row was incremented to 8 (5 + 3), not duplicated.
+        Assert.That(await factory.GetOwnedItemCountAsync(viewerId, 1), Is.EqualTo(8),
+            "Pre-existing OwnedItemEntry must be found via the ThenIncluded Item nav; " +
+            "otherwise RewardGrantService falls through to add a new row and the " +
+            "(ViewerId, ItemId) unique index throws on SaveChanges.");
+
+        // reward_list reflects the post-state total (8), not the gift delta (3).
+        using var doc = JsonDocument.Parse(bodyStr);
+        var itemEntry = doc.RootElement.GetProperty("reward_list").EnumerateArray()
+            .First(e => e.GetProperty("reward_type").GetString() == "4"
+                     && e.GetProperty("reward_id").GetString() == "1");
+        Assert.That(itemEntry.GetProperty("reward_num").GetString(), Is.EqualTo("8"),
+            "RewardNum carries the POST-STATE TOTAL — client direct-assigns it onto the " +
+            "cached count, so emitting the delta would clobber on-screen inventory.");
+    }
+
     [Test]
     public async Task GiftReceive_second_call_with_same_ids_does_not_double_grant()
     {