Simplifications

Tests/test_file_gif.py

@@ -472,7 +472,7 @@ def test_multiple_duration(self):
             except EOFError:
                 pass
 
-    def test_partially_identical_frames(self):
+    def test_identical_frames(self):
         duration_list = [1000, 1500, 2000, 4000]
 
         out = self.tempfile("temp.gif")
@@ -495,37 +495,23 @@ def test_partially_identical_frames(self):
         # Assert that the new duration is the total of the identical frames
         self.assertEqual(reread.info["duration"], 4500)
 
-    def test_totally_identical_frames(self):
+    def test_identical_frames_to_single_frame(self):
         duration_list = [1000, 1500, 2000, 4000]
 
         out = self.tempfile("temp.gif")
-
-        image_path = "Tests/images/bc7-argb-8bpp_MipMaps-1.png"
         im_list = [
-            Image.open(image_path),
-            Image.open(image_path),
-            Image.open(image_path),
-            Image.open(image_path),
+            Image.new("L", (100, 100), "#000"),
+            Image.new("L", (100, 100), "#000"),
+            Image.new("L", (100, 100), "#000"),
         ]
-        mask = Image.new("RGBA", im_list[0].size, (255, 255, 255, 0))
-
-        frames = []
-        for image in im_list:
-            frames.append(Image.alpha_composite(mask, image))
 
         # duration as list
-        frames[0].save(
-            out,
-            save_all=True,
-            append_images=frames[1:],
-            optimize=False,
-            duration=duration_list,
-            loop=0,
-            transparency=0,
+        im_list[0].save(
+            out, save_all=True, append_images=im_list[1:], duration=duration_list
         )
         reread = Image.open(out)
 
-        # Assert that all four frames were combined
+        # Assert that all frames were combined
         self.assertEqual(reread.n_frames, 1)
 
         # Assert that the new duration is the total of the identical frames

src/PIL/GifImagePlugin.py

@@ -472,9 +472,6 @@ def _write_multiple_frames(im, fp, palette):
             else:
                 bbox = None
             im_frames.append({"im": im_frame, "bbox": bbox, "encoderinfo": encoderinfo})
-    # see: https://github.com/python-pillow/Pillow/issues/4002
-    if len(im_frames) == 1 and "duration" in im_frames[0]["encoderinfo"]:
-        im.encoderinfo["duration"] = im_frames[0]["encoderinfo"]["duration"]
 
     if len(im_frames) > 1:
         for frame_data in im_frames:
@@ -492,6 +489,9 @@ def _write_multiple_frames(im, fp, palette):
                 offset = frame_data["bbox"][:2]
             _write_frame_data(fp, im_frame, offset, frame_data["encoderinfo"])
         return True
+    elif "duration" in im.encoderinfo:
+        # Since multiple frames will not be written, add together the frame durations
+        im.encoderinfo["duration"] = sum(im.encoderinfo["duration"])
 
 
 def _save_all(im, fp, filename):