Do not save duplicates when duplicate sizes are supplied

Tests/test_file_ico.py

@@ -1,4 +1,5 @@
 import io
+import os
 
 import pytest
 
@@ -70,6 +71,20 @@ def test_save_to_bytes():
         )
 
 
+def test_no_duplicates(tmp_path):
+    temp_file = str(tmp_path / "temp.ico")
+    temp_file2 = str(tmp_path / "temp2.ico")
+
+    im = hopper()
+    sizes = [(32, 32), (64, 64)]
+    im.save(temp_file, "ico", sizes=sizes)
+
+    sizes.append(sizes[-1])
+    im.save(temp_file2, "ico", sizes=sizes)
+
+    assert os.path.getsize(temp_file) == os.path.getsize(temp_file2)
+
+
 @pytest.mark.parametrize("mode", ("1", "L", "P", "RGB", "RGBA"))
 def test_save_to_bytes_bmp(mode):
     output = io.BytesIO()

src/PIL/IcoImagePlugin.py

@@ -51,7 +51,7 @@ def _save(im, fp, filename):
         else True,
         sizes,
     )
-    sizes = list(sizes)
+    sizes = set(sizes)
     fp.write(struct.pack("<H", len(sizes)))  # idCount(2)
     offset = fp.tell() + len(sizes) * 16
     bmp = im.encoderinfo.get("bitmap_format") == "bmp"