To copy an item, just open it then click on the "..." icon in the top right-hand corner, or click the vertical "..." icon on any item to open the quick edit menu.
Select "Duplicate" from the dropdown menu.
Copying an item in this way will duplicate it exactly. The copy of the item will appear below the original in each view.
Copying in bulk
To copy multiple items at once, first select all the items you want to copy by holding down the Alt key while clicking on the items, or by using the checkboxes in table view, or the Ctrl/Cmd+A shortcut.
Next, click on where it shows the number of items selected. Click "Duplicate" to copy the items within the collection, or "Copy items to collection" to copy them into another collection.
If you want to copy the items to another collection, make sure to map the fields correctly by choosing which field should copy to which in the new collection.