How do I stop a signature being repeated in an email conversation?

There may be times where it is only appropriate to insert a given signature once. For example, if you are applying a signature to outgoing messages, you might want it to be added the first time that a message is sent, but prefer not to add it at the end of any subsequent replies if the email turns into a conversation. This is simple to achieve by selecting the don't add signature if already present check box on the signature options tab: