From 3e37a919b136d35447bde7694ecf579a2096b163 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Sat, 28 Jan 2023 22:43:04 +1100
Subject: [PATCH] Prevent register_open from adding duplicates to ID

---
 Tests/test_image.py | 11 +++++++++++
 src/PIL/Image.py    |  3 ++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/Tests/test_image.py b/Tests/test_image.py
index d261638f941..ad3346b5a58 100644
--- a/Tests/test_image.py
+++ b/Tests/test_image.py
@@ -398,6 +398,17 @@ def test_alpha_inplace(self):
         with pytest.raises(ValueError):
             source.alpha_composite(over, (0, 0), (0, -1))
 
+    def test_register_open_duplicates(self):
+        # Arrange
+        factory, accept = Image.OPEN["JPEG"]
+        id_length = len(Image.ID)
+
+        # Act
+        Image.register_open("JPEG", factory, accept)
+
+        # Assert
+        assert len(Image.ID) == id_length
+
     def test_registered_extensions_uninitialized(self):
         # Arrange
         Image._initialized = 0
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index 833473f789d..ad0d25add90 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -3406,7 +3406,8 @@ def register_open(id, factory, accept=None):
        reject images having another format.
     """
     id = id.upper()
-    ID.append(id)
+    if id not in ID:
+        ID.append(id)
     OPEN[id] = factory, accept