which way is the best to copy object on stage in the corel draw ? use shortcut key , transform dialog box , or copy paste?

All options are up to your preference...
But I would prefer selecting object need to be copied and SIMPLY PRESSING A PLUS SIGN (+) ON MY NUMPAD.
It makes a copy of all selected objects on above the selection.
Just move it aside using your mouse or keyboard arrow keys, and your copied elements are there.
:) This copy paste can change the location of elements and could paste it on 0,0 location on your page.
Transform dialogue box is good for people comfortable working most of the things with the help of Mouse.

If i want to copy object at the Same Page , So i will simply hold the object drag and right clik to copy . if i need many copies then after leave the object at the place where i needed and then i will use the short key Ctrl+R .