To duplicate an existing messaging template, start on your homepage and click into Settings.
Next, click Automated messages from the menu on the left.
Once inside your messages page, find the template you want to duplicate and click the three-dot button at the far-right side of its row.
The duplicate will then appear with the same name as it's mother template, but with "- Copy" next to it. Once you've made your duplicate template, you can edit the name and contents as you please!
For more information on editing a messaging sequence, see Editing Message Sequences.