我正在使用act-as-taggable-on gem。我似乎做的每件事都正确，但每当我尝试创建一个标签时，我都会收到错误：
PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "tags_pkey"
DETAIL: Key (id)=(2) already exists.
: INSERT INTO "tags" ("name") VALUES ($1) RETURNING "id"
(0.3ms) ROLLBACK
Completed 500 Internal Server Error in 973ms
ActsAsTaggableOn::DuplicateTagError - 'Untag' has already been taken: