Only combine PNG frames if the combined duration is still writable

Tests/test_file_apng.py

@@ -562,6 +562,22 @@ def test_apng_save_large_duration(tmp_path: Path) -> None:
     with pytest.raises(ValueError, match="cannot write duration"):
         im.save(test_file, save_all=True, append_images=[im2], duration=65536000)
 
+    # Do not merge identical frames if the combined duration cannot be written
+    im.save(
+        test_file,
+        save_all=True,
+        append_images=[im.copy()],
+        duration=[32768000, 32768000],
+    )
+
+    with Image.open(test_file) as reloaded:
+        assert isinstance(reloaded, PngImagePlugin.PngImageFile)
+        assert reloaded.n_frames == 2
+        assert reloaded.info["duration"] == 32768000
+
+        reloaded.seek(1)
+        assert reloaded.info["duration"] == 32768000
+
 
 def test_apng_save_disposal(tmp_path: Path) -> None:
     test_file = tmp_path / "temp.png"

src/PIL/PngImagePlugin.py

@@ -1261,8 +1261,13 @@ def _write_multiple_frames(
                     and prev_blend == encoderinfo.get("blend")
                     and "duration" in encoderinfo
                 ):
-                    previous.encoderinfo["duration"] += encoderinfo["duration"]
-                    continue
+                    new_duration = (
+                        previous.encoderinfo["duration"] + encoderinfo["duration"]
+                    )
+                    delay = Fraction(new_duration / 1000).limit_denominator(65535)
+                    if delay.numerator <= 65535:
+                        previous.encoderinfo["duration"] = new_duration
+                        continue
             else:
                 bbox = None
             im_frames.append(_Frame(im_frame, bbox, encoderinfo))