Mating in ducks in notoriously coercive. Males will force themselves on females.
In some species, 40 % of mating are forced.
However, only 2-5 % of offspring are from these forced matings, suggesting the females have a trick up their sleeves.
Image from https://www.smithsonianmag.com/science-nature/why-have-female-animals-evolved-such-wild-genitals-180979813/
The male duck has a corkscrew penis that explodes out at 75pmh (see it here!). The female duck vagina is also corkscrew-shaped but spirals in an opposing direction to the penis.
This makes it nearly impossible for the penis to fully enter the vagina unless the female allows it.
Vagina on the left and penis on the right with opposing spirals. Image from https://www.smithsonianmag.com/science-nature/why-have-female-animals-evolved-such-wild-genitals-180979813/
You can experience a duck's vagina in this virtual reality explorer!
See Isabell Rossellini as a female duck here!
References and Further Reading
Brennan, P. L., Clark, C. J., & Prum, R. O. (2010). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia. Proceedings of the Royal Society B: Biological Sciences, 277(1686), 1309-1314.