what size duncan glove should fit my hand

when i measured my tip of my left hand to my wrist it is somewhere between 17-18cm

you should get a medium glove

if you have fat or chubby fingers get the medium.

but for me, measurement is almost the same as yours. but i have “skinnier” fingers. so when i bought the medium, it was too big and loose for my fingers.
i should’ve definitely bought a small, but my mom can sew. so my problem is fixed.
so just a heads up for you.

my fingers aren’t skinny or chubby

Then just get a small it can’t hurt to stretch the glove out a little.