Have anyone tried to transfect a cell multiple times with same DNA. Like a cell line which is already made stable with a particular DNA can be transfected again with the same DNA which is already integrated into the genome?
Yes, it can be transfected, in principle. I'm not sure what you are trying to achieve though, and what kind of transfection you are talking about. Retroviral? Plasmid? CRISPR? The answers about what will happen will differ depending on it.