From 0bd1fbc731cb21ba92262e2fff48ff49a21c3aa3 Mon Sep 17 00:00:00 2001
From: Julien Duroure <julien.duroure@gmail.com>
Date: Tue, 17 Jun 2025 12:45:53 +0200
Subject: [PATCH] glTF exporter: Variant: Sanity checks to avoid bad duplicate
 exports

---
 scripts/addons_core/io_scene_gltf2/__init__.py      |  2 +-
 .../io_scene_gltf2/blender/com/gltf2_blender_ui.py  | 13 +++++++++++--
 .../io_scene_gltf2/blender/exp/primitives.py        | 12 ++++++++++++
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/scripts/addons_core/io_scene_gltf2/__init__.py b/scripts/addons_core/io_scene_gltf2/__init__.py
index 83070ddafff..f952c720434 100755
--- a/scripts/addons_core/io_scene_gltf2/__init__.py
+++ b/scripts/addons_core/io_scene_gltf2/__init__.py
@@ -5,7 +5,7 @@
 bl_info = {
     'name': 'glTF 2.0 format',
     'author': 'Julien Duroure, Scurest, Norbert Nopper, Urs Hanselmann, Moritz Becher, Benjamin Schmithüsen, Jim Eckerlein, and many external contributors',
-    "version": (4, 5, 41),
+    "version": (4, 5, 42),
     'blender': (4, 4, 0),
     'location': 'File > Import-Export',
     'description': 'Import-Export as glTF 2.0',
diff --git a/scripts/addons_core/io_scene_gltf2/blender/com/gltf2_blender_ui.py b/scripts/addons_core/io_scene_gltf2/blender/com/gltf2_blender_ui.py
index 67d6d9c41b1..41300447947 100644
--- a/scripts/addons_core/io_scene_gltf2/blender/com/gltf2_blender_ui.py
+++ b/scripts/addons_core/io_scene_gltf2/blender/com/gltf2_blender_ui.py
@@ -212,18 +212,27 @@ class SCENE_OT_gltf2_assign_to_variant(bpy.types.Operator):
         for mat_slot_idx, s in enumerate(obj.material_slots):
             # Check if there is already data for this slot
             found = False
+            variant_found = False
             for i in obj.data.gltf2_variant_mesh_data:
                 if i.material_slot_index == mat_slot_idx and i.material == s.material:
                     found = True
                     variant_primitive = i
+                elif i.material_slot_index == mat_slot_idx and bpy.data.scenes[0].gltf2_active_variant in [
+                        v.variant.variant_idx for v in i.variants]:
+                    # User changed the material, so store the new one (replace instead of add)
+                    found = True
+                    variant_found = True
+                    variant_primitive = i
+                    i.material = s.material
 
             if found is False:
                 variant_primitive = obj.data.gltf2_variant_mesh_data.add()
                 variant_primitive.material_slot_index = mat_slot_idx
                 variant_primitive.material = s.material
 
-            vari = variant_primitive.variants.add()
-            vari.variant.variant_idx = bpy.data.scenes[0].gltf2_active_variant
+            if variant_found is False:
+                vari = variant_primitive.variants.add()
+                vari.variant.variant_idx = bpy.data.scenes[0].gltf2_active_variant
 
         return {'FINISHED'}
 
diff --git a/scripts/addons_core/io_scene_gltf2/blender/exp/primitives.py b/scripts/addons_core/io_scene_gltf2/blender/exp/primitives.py
index a7bf2181b64..0c44dbc5a0d 100644
--- a/scripts/addons_core/io_scene_gltf2/blender/exp/primitives.py
+++ b/scripts/addons_core/io_scene_gltf2/blender/exp/primitives.py
@@ -304,14 +304,26 @@ def __gather_extensions(blender_mesh,
 
     # Material idx is the slot idx. Retrieve associated variant, if any
     mapping = []
+    variants_idx_in_use = []
     for i in [v for v in blender_mesh.gltf2_variant_mesh_data if v.material_slot_index == material_idx]:
         variants = []
         for idx, v in enumerate(i.variants):
             if v.variant.variant_idx in [o.variant.variant_idx for o in i.variants[:idx]]:
                 # Avoid duplicates
                 continue
+
+            # Avoid duplicates from a previous variant (in mapping list)
+            # This happen only before fix of #2542
+            if v.variant.variant_idx in variants_idx_in_use:
+                # Avoid duplicates
+                export_settings['log'].warning(
+                    'Variant ' + str(v.variant.variant_idx) +
+                    ' has 2 differents materials for a single slot. Skipping it.')
+                continue
+
             vari = ext_variants.gather_variant(v.variant.variant_idx, export_settings)
             if vari is not None:
+                variants_idx_in_use.append(v.variant.variant_idx)
                 variant_extension = gltf2_io_extensions.ChildOfRootExtension(
                     name="KHR_materials_variants",
                     path=["variants"],