I Have configured email-to-case. there is one support distribution group. there are three members(A,B,C) in the group and Email-to-Case address. when a customer sends an email to this distribution group Case will be created in SF. and A, B, C also gets an email. When A responds to the customer through outlook using reply all then another case is creating in SF along with reply to a customer. similarly, when B responds to the initial email sent by a customer, the same thing happens, so there are 3 cases of a similar kind. first one by a customer, the second one by A and third one by B, 2 duplicates created. this is happening only when A and B respond through outlook. how to avoid this. Thanks.
This is because they're responding directly to the customer email and their email does not yet have a Thread ID that Salesforce uses to keep emails/replies under one case. They're interacting with the email/customer outside of Salesforce and the case.
Salesforce appends an unique thread ID that you can see in Salesforce on the case when you try to respond.
I'd advise you to change your workflow. The point of Email-to-case is that the emails go into Salesforce and your end-users don't need to be cc'd on them. From Salesforce, on the case, is where your end-users (in your example: A, B, and C) need to respond to the email. This will ensure their email response contains the Thread ID, and that everything stays under the same case.