Only compare to previous frame when checking for duplicate GIF frames while saving

Tests/test_file_gif.py

@@ -677,6 +677,24 @@ def test_dispose2_background(tmp_path):
         assert im.getpixel((0, 0)) == (255, 0, 0)
 
 
+def test_dispose2_background_frame(tmp_path):
+    out = str(tmp_path / "temp.gif")
+
+    im_list = [Image.new("RGBA", (1, 20))]
+
+    different_frame = Image.new("RGBA", (1, 20))
+    different_frame.putpixel((0, 10), (255, 0, 0, 255))
+    im_list.append(different_frame)
+
+    # Frame that matches the background
+    im_list.append(Image.new("RGBA", (1, 20)))
+
+    im_list[0].save(out, save_all=True, append_images=im_list[1:], disposal=2)
+
+    with Image.open(out) as im:
+        assert im.n_frames == 3
+
+
 def test_transparency_in_second_frame(tmp_path):
     out = str(tmp_path / "temp.gif")
     with Image.open("Tests/images/different_transparency.gif") as im:

src/PIL/GifImagePlugin.py

@@ -565,6 +565,16 @@ def _write_single_frame(im, fp, palette):
     fp.write(b"\0")  # end of image data
 
 
+def _getbbox(base_im, im_frame):
+    if _get_palette_bytes(im_frame) == _get_palette_bytes(base_im):
+        delta = ImageChops.subtract_modulo(im_frame, base_im)
+    else:
+        delta = ImageChops.subtract_modulo(
+            im_frame.convert("RGB"), base_im.convert("RGB")
+        )
+    return delta.getbbox()
+
+
 def _write_multiple_frames(im, fp, palette):
 
     duration = im.encoderinfo.get("duration")
@@ -598,6 +608,12 @@ def _write_multiple_frames(im, fp, palette):
             if im_frames:
                 # delta frame
                 previous = im_frames[-1]
+                bbox = _getbbox(previous["im"], im_frame)
+                if not bbox:
+                    # This frame is identical to the previous frame
+                    if encoderinfo.get("duration"):
+                        previous["encoderinfo"]["duration"] += encoderinfo["duration"]
+                    continue
                 if encoderinfo.get("disposal") == 2:
                     if background_im is None:
                         color = im.encoderinfo.get(
@@ -606,21 +622,7 @@ def _write_multiple_frames(im, fp, palette):
                         background = _get_background(im_frame, color)
                         background_im = Image.new("P", im_frame.size, background)
                         background_im.putpalette(im_frames[0]["im"].palette)
-                    base_im = background_im
-                else:
-                    base_im = previous["im"]
-                if _get_palette_bytes(im_frame) == _get_palette_bytes(base_im):
-                    delta = ImageChops.subtract_modulo(im_frame, base_im)
-                else:
-                    delta = ImageChops.subtract_modulo(
-                        im_frame.convert("RGB"), base_im.convert("RGB")
-                    )
-                bbox = delta.getbbox()
-                if not bbox:
-                    # This frame is identical to the previous frame
-                    if encoderinfo.get("duration"):
-                        previous["encoderinfo"]["duration"] += encoderinfo["duration"]
-                    continue
+                    bbox = _getbbox(background_im, im_frame)
             else:
                 bbox = None
             im_frames.append({"im": im_frame, "bbox": bbox, "encoderinfo": encoderinfo})