Fix bug when merging identical images to GIF with a list of durations

Tests/test_file_gif.py

@@ -495,6 +495,26 @@ def test_identical_frames(self):
         # Assert that the new duration is the total of the identical frames
         self.assertEqual(reread.info["duration"], 4500)
 
+    def test_identical_frames_to_single_frame(self):
+        for duration in ([1000, 1500, 2000, 4000], (1000, 1500, 2000, 4000), 8500):
+            out = self.tempfile("temp.gif")
+            im_list = [
+                Image.new("L", (100, 100), "#000"),
+                Image.new("L", (100, 100), "#000"),
+                Image.new("L", (100, 100), "#000"),
+            ]
+
+            im_list[0].save(
+                out, save_all=True, append_images=im_list[1:], duration=duration
+            )
+            reread = Image.open(out)
+
+            # Assert that all frames were combined
+            self.assertEqual(reread.n_frames, 1)
+
+            # Assert that the new duration is the total of the identical frames
+            self.assertEqual(reread.info["duration"], 8500)
+
     def test_number_of_loops(self):
         number_of_loops = 2
 

src/PIL/GifImagePlugin.py

@@ -489,6 +489,11 @@ 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 and isinstance(
+        im.encoderinfo["duration"], (list, tuple)
+    ):
+        # 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):