A separate new issue will be created is correct because the client sent a fresh, separate email rather than replying to the original message and ticketing systems treat new, standalone messages as new requests.

Many ticketing systems rely on email headers such as Message ID and In Reply To or on threaded reply behavior to associate incoming mail with an existing ticket. When those headers are absent or when the mail is a new message the system cannot match it to the original ticket and so it creates a new issue.

A new subtask will be added to the existing ticket is incorrect because a separate incoming email will not automatically become a subtask unless explicit automation or manual action links it that way.

A new comment will be appended to the current issue is incorrect because appending a comment requires the incoming mail to be recognized as a reply to the ticket thread and that recognition depends on headers or reply routing not on a fresh standalone message.

Nothing will happen since the message content is identical is incorrect because ticket systems do not usually use identical message content to decide whether to ignore or merge messages and identical text will not prevent a new issue from being created.

A new issue will be created and linked to the existing ticket is incorrect because automatic linking of separate issues is not the default behavior. Some platforms can be configured to create links for related tickets but a plain separate email will normally only create a separate issue.

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