This plant has been growing in my pond for quite a time now. It multiplies in large numbers rapidly. Some told me it is duckweed or is it something else?
That looks very much like a duckweed.
The multiple roots rule out most genera within this group and so this is probably a Spirodela.
The apparently relatively large size and red undersides are consistent with this being Spirodela polyrhiza.
Picture by Christian Fischer available under Creative Commons Attribution-Share Alike 3.0 Unported.
Not duck weed. It is Salvinia species very easy to grow ( invasive). I believe it is illegal to introduce in some US states. Koi /carp will eat duck weed.There are a few similar species of salvinia , unfortunately references will give the same name for different plants. My book ( Aquarium Plants ; Barry Jones, Tetra press) calls it auriculata which I believe is wrong. Auriculata is also called giant salvinia and looks different ( see comments @ tyersome ). I have grown Auriculata but not the salvinia that you have.