Go to My Trints and Stories list
Click on the checkbox next to the Trint or Caption file
Click on the Duplicate button in the top toolbar
You are not able to duplicate files in the Shared With Me list at the moment.
You are not able to duplicate Stories at the moment.
If you would like to duplicate a file from the Recent Trints list, please find it in your My Trints and Stories list and duplicate from there.