Duns come in a lot of different shades, and their face and legs are usually a darker version of their body coat, along with the dorsal stripe. Their manes and tails also show the stripe going through them. Plus the leg barring, but I think buckskins can have that too. Duns also usually have a shoulder stripe/ patch of darker hair.
According to AQHA: duns have the dorsal stripe and / or the cruz mark across the shoulders and 'zebra' markings on the legs.
Buckskins are similarly colored but have NO dorsal stripe.
Both Duns and buckskin have black 'points' on the legs
Grullos (masculine) and Grullas (feminine) are similar but seperate as they carry the black gene as well as the dun gene. (black dun) Grullo/grulla display primitive markings ( some intensely so) on the neck, legs, face and ears. and/or
Buckskin and dun are both caused by dilution genes that fade the base color of the horse. Dun is a dominant gene. One copy of the gene gives all characteristics which are known as primitive markings. This always includes a dorsal stripe, and may also show zebra striping on the legs, cobwebbing on the face, a transverse stripe on the wither and darker tipped ears. A red horse with dun is known as a red dun, a bay horse with dun is often mistaken for a buckskin, and a black horse with dun is known as a grulla.
Buckskin is caused by the Cream gene which is an incomplete dominant. One copy of the cream gene will turn a red horse into a palomino. Two copies of the gene will turn it into a cremello. One copy on bay will make it a buckskin, two copies on bay makes it a perlino. One cream on black is known as smoky black and two is known as smoky cream.
So I guess it all boils down to genetics and which dilution they have.