Conversation 创建对话设置了 unique 参数，但对话仍不唯一

{
"results": [
    {
        "c": "2383702678", 
        "attr": {
            "user_agent": "BaseGrid/Micro Server"
        }, 
        "objectId": "57358104c4c9710060fa1f25", 
        "lm": {
            "iso": "2016-05-18T13:31:33.541Z", 
            "__type": "Date"
        }, 
        "m": [
            "2383702678", 
            "2383702607"
        ], 
        "mu": [ ], 
        "updatedAt": "2016-05-19T10:56:27.365Z", 
        "unique": true, 
        "createdAt": "2016-05-13T07:23:48.719Z"
    }, 
    {
        "c": "2383702678", 
        "attr": {
            "user_agent": "Hahaha/7.0.0/en (x86_64; iPhone OS9.3; en en_US)"
        }, 
        "objectId": "573da671a2a38530228e0ec1", 
        "lm": {
            "iso": "2016-05-19T11:41:56.302Z", 
            "__type": "Date"
        }, 
        "m": [
            "2383702607", 
            "2383702678"
        ], 
        "mu": [ ], 
        "updatedAt": "2016-05-19T11:41:56.325Z", 
        "unique": true, 
        "createdAt": "2016-05-19T11:41:53.485Z"
    }
]

}

57358104c4c9710060fa1f25 是通过 /1.1/classes/_Conversation REST API 先创建
573da671a2a38530228e0ec1 是通过 iOS 客户端后创建，设置了 unique 相关参数，但对话还是会重复多出一条

麻烦帮忙定位下原因，谢谢。