the first thing you need to do is making sure that what ever you right clicked on is the selected cell; you can do this by handling "CellMouseDown" event on the gridview and select the cell in programmatic fashion like
datagridview1[e.columnindex,e.rowindex].selected=true;
and hence there won't even be a need for you to validate if the cell right clicked on and the selected one are different cells.
I hope that helped
good luck