duplicate messages when using send batch of messages

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with the firebase tag.
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of the above categories, reach out to the personalized
[Firebase support channel](https://firebase.google.com/support/).

### [REQUIRED] Step 2: Describe your environment

* Operating System version: window 10 (19045)
* Firebase SDK version: 2.4.0 with VS2019
* Firebase Product: messaging (auth, database, storage, etc)
* .NET version: 4.6.1
* OS: windows dotnet

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

When I send a list of messages using sendEachAsync method with data read all info from my DB (about 10000 users) and loop each 500 record and send batch then received duplicate messages.

I using code follow in doc:
https://firebase.google.com/docs/cloud-messaging/send-message

#### Relevant Code:
if (FirebaseApp.DefaultInstance == null)
{
FirebaseApp.Create(new AppOptions()
{
Credential = GoogleCredential.FromFile(Server.MapPath(ConfigurationManager.AppSettings["FCMKey"]))
});
}

var messages = new List()
int cnt = 0;
foreach (DataRow item in iResult.CursorData.Rows)
{
cnt++;

messages.Add(new Message()
{
Notification = new Notification()
{
Title = item["title"].ToString(),
Body = item["DESCRIPTION"].ToString()
},
Token = item["GCMID"].ToString()
});

if (cnt == 500 || cnt==iResult.CursorData.Rows.Count)
{
try
{

string Obj = fcm.SendMultiNotiV2(messages);
messages.Clear();

}
catch (Exception ex)
{
messages.Clear();
}
finally
{
cnt = 0;
messages.Clear();

}
}
}
####