We do not have the direct feature of duplicating a singe question/slide from a presentation. However, you can duplicate a presentation that contains the question/slide that you want to copy over, and then delete the other questions that you do not need.
This is where you can find the duplicate button:
If you wish to have the feature of copying a question/slide from one presentation to the other, do head over to our product forum to up-vote on this feature! It will then help us prioritize even more on the features to work on.