Copy Objects on the Diagram Window

On the diagram window, you can copy objects using the CTRL+drag and drop key sequence.

You can copy individual attributes or columns when you select them and use CTRL+drag and drop. If you simply select and drag attributes or columns, they are moved not copied.
When you copy objects using CTRL+drag and drop, the newly created object overlaps the object that you copied. Ensure that you select and drag the new object to move it.

To copy objects on the diagram window

Open a diagram and select the objects that you want to copy.
Click Edit, Copy.
The objects are copied.
Click Paste on the Edit menu.
A message appears informing you that the objects are pasted as new objects in the target model. The new objects appear in the Model Explorer. The new objects are pasted on top of the items that you selected to copy.
Click the objects you selected to copy and drag onto a blank area on the diagram window. Change the names of all copied model objects to enforce unique naming in the model. If you are working in a Logical/Physical model, change the name on the logical side, to automatically change the name on the physical side. If you rename the object on the physical side, the change is not automatically reflected on the logical side.

Cut or copied objects remain on the clipboard even after you paste them into another location. This is convenient if you want to paste more than one copy. But, if you have a large copy selection on the clipboard, it can take up too much memory. To free up memory, after you finish copying and pasting, choose a simple, single entity and copy it to the clipboard to replace the large copy set.

More information:

Overview of Copying Objects

Copy Attributes or Columns on the Diagram Window