Check input value is duplicated when quantile queue is full (#7091)
Check input value is duplicated when quantile queue is full (#7091)

Co-authored-by: Taewoo Kim <61b662e787374c7df1bc7cbd5e922f2150ef4119@layer6.com>
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost