One of the easiest ways to do this is simply select the row (discussion below about selecting) and hold the "Ctrl" + "D" ("D" is for "Duplicate"). This will duplicate the row and place it directly under it. Also, you can use "Duplicate" by clicking Edit>Duplicate
Selecting rows can sometimes be tricky. If I use the mouse, I always try to confirm in the Object and/or Hierarchy Pallet.The mouse can select the row (arrow with an underscore) or all the objects in the row (arrow no underscore)
Here are 3 ways:
The mouse can select the row (arrow with an underscore) or all the objects in the row (arrow no underscore). You move the cursor just to the left of the table and the arrow appears.
One of the best ways I find is selecting the row in the Hierarchy Pallet.
Select an object in the row and on the tool bar click Table>Select>Row
I avoid pasting anything if I can help it. If you use flowed forms and if you paste a lot, and If you aren't careful, you'll end up with a corrupt form. Designer is very unpredictable about where it will paste. If you have a large form, it can take a minute or two to track down a pasted object's location. Then you have to move it to where you want it.
Better to use "Duplicate". That usually means it will be next to the item you're duplicating. If you copy and paste, paste, paste repeatedly too fast, You can really mess things up. Every Paste command takes longer than you think to execute. Wait at least 2 seconds before doing anything after pasting. Otherwise, you'll interpt Designer while it's busy writing the new XML. Yes, Designer is still busy doing work even after you see the new item appear. Be patient.