it sounds like maybe you are saying that when someone replies to an email another ticket is created rather than a comment being added? If also seems that this is a Service Desk question rather than a Jira Software question? If so, maybe the following will help you?
If a customer creates a ticket by email and they happen to CC others then if the others reply to the email that came from the original customer then indeed multiple tickets will be created. This is because JSD sees this as an unrelated email. When JSD processes the email it will open a ticket and notify back w/ the JSD ticket ID and underlying email header info. If anyone replies to the email from JSD then a comment will be added to the original issue.
There are a number of threads in the Community that discuss this very same issue so you may want to search for related threads.
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