Here is a highly voted (votes artificially inflated by, of course, a HubSpot “techincal error” lol) thread asking the same question: https://community.hubspot.com/t5/HubSpot-Ideas/Cloning-a-Deal/idi-p/28845
It is “not currently planned.”
It it helpful to comment and upvote on as many threads as possible to emphasize the importance of the need of the feature discussed, so please copy your comment and add an upvote there as well!