The Japanese, with their obsequious smile, are neat and trim in white duck, while their women walk a step or two behind them, in native dress, with a baby on their backs.
(W.S. Maugham; Honolulu)
1. a heavy, plain-weave cotton fabric for tents, clothing, bags, etc., in any of various weights and widths.
(Random House Webster's Unabridged Dictionary)
2. ducks plural : light clothes made of duck ; especially : trousers made of such material
(Webster's Third New International Dictionary of the English Language)
Why didn't W.S. Maugham use 'ducks' in the original sentence?
Its common with fabric names to apply or omit an s with no great effect on the sense - except that the plural can be used to refer to a range of fabrics or clothes: 'After her marriage she was always wearing silks and satins'.
I think I have mentioned this before, suprunp (if not, I intended to): very few, if any, writers produce what would be universally accepted as 'perfect English', because perfect English does not exist. Writers on grammar in the past (Cobbett, Fowler and Vallins spring to mind) have delighted in discovering examples of supposedly 'bad English' in the works of 'good' writers.
Even if writers had a style guide beside them as they wrote, and checked every single word and construction before they wrote them down, they would not satisfy the writer of a different style guide.
I've never heard of "duck" as a type of clothing, so can offer you no advice about how to use it properly.