Duckweed is a pain when it gets in between the leaves of your marginal plants - you can use a wide flat spray on your hose to move it out, but if there's no un-planted corner to aim for, most of it will end up among other marginals - especially the ones you have just cleaned out...
I put in a lower (40cm) pond with a waterfall I could fix a sieve under - I only had room for 120 x 60cm, so it's easy to clean - though it breaks the normal rules about the largest pond being the lower one - but I only use it when I'm washing duckweed out of the main upper pond or when I want to sit near the waterfall.
The flow plus the flat spray are very, very efficient - it only takes me 20 minutes to clean out everything visible so it takes weeks to become a problem again.
Another solution I've used is to plant marginals in big pots inside the pond with their rims above the water. Good for controlling irises, sagittaria etc.
They look good: hard-glazed ones are best - they need wet strength - a pattern round the rim looks nice. I even grew a manageable but still impressive 100cm+ tall G.manicata this way.
Cheers.