hi,
Could any one please tell me how to perform dual operation for a single 
clock pluse(rising edge,falling edge/on,off) in VHDL.
thanks in advance.

What do you need this for? DDR Ram or something likewise?

Yes it is for DDR RAM based application.

Then the only place you need both edges is the IO cell.
And in the documentation for the IO pins you can find the documentation, 
how to implement a DDR interface. Indeed there is no real "both edge 
flipflop", but two seperate "rising-edge-flipflops", and one of them is 
supplied with the inverted clock. And this DDR cell is usually not 
described in VHDL, but it is a component (black box) provided from the 
manufaturer of the FPGA to be instantiated with a VHDL template...

Thanks for your reply.