C-c C-x c (org-clone-subtree-with-time-shift)
Clone a subtree by making a number of sibling copies of it. You are prompted for the number of copies to make, and you can also specify if any timestamps in the entry should be shifted. This can be useful, for example, to create a number of tasks related to a series of lectures to prepare. For more details, see the docstring of the command org-clone-subtree-with-time-shift.