在mongodb中我有一个集合，其中数组有重复的条目，比如{"_id":ObjectId("57cf3cdd5f20a3b0ba009777"),"Chat":6,"string":["1348157031Riyadh","548275320MohammadSumon","1348157031Riyadh","548275320MohammadSumon","1348157031Riyadh","1348157031Riyadh"]}我需要删除重复的数组并只保留唯一的数组值，如下所示。{"_id":ObjectId("57cf3cdd5f20a3b0ba009777"),"Chat":