I have duckweed...and a fastish flow in my 53 gallon.
The duckweed does not last long but not for the usual reason
If the duckweed dares to venture into the flow of either filter, (which is a big misjudgement on the part of the duckweed), the filters will fling the (either very brave or very stupid) duckweed into a stream of high speed somersaults that a gymnast would be proud of
Unfortunately during this gymnastic display by the duckweed, it sometimes slaps a fish or two around the face (something like slapping a human face with wet lettuce)
The fish being slapped by the soggy and dazed duckweed are generally none too impressed by that behaviour, so they chase the duckweed down.....
I got rid of it from two tanks a few months ago, it took a whole afternoon for each tank though.
Remove all floating plants to a bucket of old tank water, then net out every single duckweed plant. The ones stuck in the corner/round the heater and filter cables can be removed one by one with tweezers. Check every nook and cranny as just one plant missed will cause another invasion.
Swoosh gently the floating plants in the bucket and move them one by one to a bucket of warm dechlorinated water - I had to do this with frogbit and red root floater - the RRF took ages. Swoosh them in there and move them to another bucket of warm dechlorinated water. Swoosh again and return them to the tank. Check every day for duckweed that escaped and remove every plant you find with tweezers.
It's a lot of work but it can be done. I've been duckweed free from a couple of weeks after I did this - I did keep finding the odd plant or two for a couple of weeks, but no more since.