Does that top 20 percentile rule applies for repeaters also or only to those who give their boards/+2 in 2013..?

It applies for everybody including repeaters. Note that IIT doesnt consider board marks. Other institutions like NIT and all consider 40% board marks, 30% IIT main and 30% IIT advanced. All India - only one exam for engineering and for getting admission in NIT and all, you have to be in the top 20 percentile of your board.