If the infestation covers the whole pond you are in trouble. It's almost impossible to get rid of it without the use of chemicals. Koi like to nibble on it but will only control the weed not get rid of it. Manually removing it will also only partially control it as it's nearly impossible to remove all of it.
The best thing to do is to buy an approved duckweed herbicide to eliminate the current problem. And then after that to prevent further infestations keep the water nutrients low.
Duckweed thrives on nutrients, so reducing the nutrients in the pond and aerating using either a small waterfall or fountain helps a lot.