diff --git a/pkg/models/kanban_task_bucket.go b/pkg/models/kanban_task_bucket.go
index c8794201e..0b2b06eaf 100644
--- a/pkg/models/kanban_task_bucket.go
+++ b/pkg/models/kanban_task_bucket.go
@@ -152,6 +152,13 @@ func updateTaskBucket(s *xorm.Session, a web.Auth, b *TaskBucket) (err error) {
 				if err != nil {
 					return err
 				}
+				// If the task is already in the default bucket, skip the
+				// upsert — MySQL's UPDATE returns 0 affected rows when
+				// the value is unchanged, which would make upsert fall
+				// through to INSERT and hit the unique constraint.
+				if b.BucketID == oldTaskBucket.BucketID {
+					updateBucket = false
+				}
 			}
 		}