tso: "split 0 2 2 2" generate duplicate keyspaces in the keyspace list of the group

## Enhancement Task
root@serverless-cluster-pd-0:/# ./pd-ctl keyspace-group split 0 2 2 2
Success!

{"id":2,"user-kind":"basic","members":[{"address":"http://pd-tso-server-0.tso-service.tidb-serverless.svc:2379","priority":200},{"address":"http://pd-tso-server-1.tso-service.tidb-serverless.svc:2379","priority":300}],"**keyspaces":[2,2]**}