Not necessarily duds. I think there may be more variation in SL cleats these days, and whatever the packaging may say, I suspect it's increasingly difficult to get anything other than copies.
I was an early adopter of the original LOOK pedals in the mid 1980s, and have stuck with them ever since. My first ones have long since departed, but I still have three working pairs:
From left to right: Shimano PD-6401 "Tricolour" from about 1990; a LOOK-branded unknown model, about 15 years old; Shimano PD-R600, bought secondhand earlier this year, unknown age but believed to be much younger than the other two.
For some years now, whenever I've replaced cleats I've found it necessary to pair them with the pedals that work best; hence my belief that cleats are more variable than they used to be.
Of course, if you don't have multiple sets of pedals this doesn't help so much, but they can be found. The R600s cost me a fiver from a shop in Bath that sells recycled stuff.
There is another issue. Note that on the older two examples, the body of the pedal extends upwards either side of the hinged part. Increasingly often, as in the
Halfords example linked above, new cleats have a raised platform at either end of the back edge, which I've found makes them very hard to unclip from pedals of the older style. The left-hand Tricolour seems to have found it's own solution:
Although it's easy to see how this pedal/cleat combination could cause a problem when unclipping, I think it could also inhibit clipping in as well if the body of the pedal doesn't sit happily in the gap at the back of the cleat.