Remove duplicate bug in CHANGELOG.md (#1991)

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b91b0b8..331d623 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,7 +11,7 @@
 * Fix wrong alpha plane deallocation when decoded tile pixel format does not
   match reconstructed output image pixel format (b/320234262).
 * Fix identical chunk skipping optimization when writing animation data
-  (b/321189607 and b/321189607).
+  (b/321189607).
 * Fix ID selection for artificial grid alpha item when decoding a grid of tiles
   which each have an associated auxiliary alpha image item
   (https://crbug.com/oss-fuzz/65657).