Hi,
Another potential cause of "off" tasting duck eggs can be to do with genetics and diet - it is not as well understood as it is with brown layer hens, but predisposition to incomplete digestion of something called "TMA" (science nerds, there are scientific papers available online!) can make your duck eggs (and chicken eggs) taste off if the bird is genetically predisposed to misdigest certain components of feed.
So, if it is not illness, then it may be that a duck or two this year are a bit predisposed to that. Solutions include adjustment of the diet (reducing choline supplements in diet, if present, or steering clear of highly predisposing feed elements like rapeseed oil) or identification of the duck - in some chickens, you can smell the same smell on their breath, so you could try that with the ducks. I would get both a man and a woman to smell, if possible, as the ability to detect some compounds is sometimes sex-linked in humans!
If you come across a single duck or couple of ducks with the same smell as the eggs on their breath, you could briefly isolate them and check the eggs to make sure it is them. Then work out if you can change the diet, or if you need to do something else with the ducks.
Please be aware that the smelling test is not 100% accurate - if you smell a couple, and separate them, you can't swear you've missed one in the main flock I'm afraid, but it would give you a rough diagnosis of what the problem actually is, if it is not illness.
For illness with no obvious outward signs, weighing your suspect ducks weekly can indicate who in the flock might be ill, as they are likely to be losing weight before they start to "look" unwell.
As I say, less is known about this in ducks than in chickens, due to less research being carried out, but these are options for you to start the detective work!
Oh, and if you are sniffing duck faces (or any type of poultry face) please do think about appropriate protection for the eyes and/or gentle but effective restraint of the head...
(PS if you have recently preserved or creosoted anywhere, or have any environmental smells, that is also a possibility - I once had a chap who got badly tainted eggs because they sat in a newly creosoted shed for a few hours after washing, to dry - the shed-smell got into them!)
C