[{"mystring":"xxxyzzxcdv"}]

db.collection.aggregate([{"$set":{"mystring":{"$getField":{"field":"string","input":{"$reduce":{"input":{"$range":[0,{"$strLenCP":"$mystring"}]},"initialValue":{"previous":[],"string":""},"in":{"$let":{"vars":{"cur_char":{"$substrCP":["$mystring","$$this",1]}},"in":{"$cond":[{"$in":["$$cur_char","$$value.previous"]},"$$value",{"previous":{"$concatArrays":["$$value.previous",["$$cur_char"]]},"string":{"$concat":["$$value.string","$$cur_char"]}}]}}}}}}}}}])