This page describes how to clone an epic with all the associated issues in the same project. Instead of creating a new epic and all the child issues again, you can clone the epic using the Copy issue in epic clone option.
To clone an epic with all the child issues from the same project: (Note: this example assumes that the 'Task' issue type is excluded from cloning)
In this example, there is an epic within the ZCLONE project with a subtask and four associated issues:
Perform a clone operation with the following clone options selected:
Click Create to create the cloned epic. The parent epic is now cloned to a new epic with the subtask and all the child issues cloned (except for the issue that had Task as the issue type). Refer to the following image: