The only way that you could do this in the past is by asking the Support Chat to create a copy of the form in question, and then they could do it for you. If you were creating a copy from your Sandbox site, they couldn’t replicate the form logic. I’m not sure if they are able to replicate the form logic if they are making a copy of a report on your active site, but I would assume probably not, so you’ll most likely still have to manually recreate the form logic for the replicated form if there is any. Now that they have changed their Support Chat, I’m not sure if this is still something that they will do for you, but you can ask! (:
The suggestion that we should be able to use our own forms as templates has been on their Ideas Portal for years with no follow-up, so I don’t think that we’re going to get the ability to do that anytime soon. I agree that it would be nice if we could do it ourselves, though!