diff --git a/Tests/test_image_convert.py b/Tests/test_image_convert.py
index bbe9267d6..01bcbb95f 100644
--- a/Tests/test_image_convert.py
+++ b/Tests/test_image_convert.py
@@ -155,6 +155,12 @@ def test_trns_RGB(tmp_path):
     assert "transparency" not in im_p.info
     im_p.save(f)
 
+    im = Image.new("RGB", (1, 1))
+    im.info["transparency"] = im.getpixel((0, 0))
+    im_p = im.convert("P", palette=Image.ADAPTIVE)
+    assert im_p.info["transparency"] == im_p.getpixel((0, 0))
+    im_p.save(f)
+
 
 def test_gif_with_rgba_palette_to_p():
     # See https://github.com/python-pillow/Pillow/issues/2433
diff --git a/src/PIL/ImagePalette.py b/src/PIL/ImagePalette.py
index 236a44e37..f762d3032 100644
--- a/src/PIL/ImagePalette.py
+++ b/src/PIL/ImagePalette.py
@@ -53,10 +53,12 @@ class ImagePalette:
         self._palette = palette
 
         mode_len = len(self.mode)
-        self.colors = {
-            tuple(self.palette[i : i + mode_len]): i // mode_len
-            for i in range(0, len(self.palette), mode_len)
-        }
+        self.colors = {}
+        for i in range(0, len(self.palette), mode_len):
+            color = tuple(self.palette[i : i + mode_len])
+            if color in self.colors:
+                continue
+            self.colors[color] = i // mode_len
 
     def copy(self):
         new = ImagePalette()