Unable to buy more than 1 boot enchantment with the same type of boot with multiple boots

I am unable to buy enchants for more than 1 boot if I have more than 1 of the same type of boot

For example I have 3 Sorcerers Boots (the magic pen stacks)

I love my full pentration Velkoz 3sorcboots 1voidstaff liandrys with either sightstone or a haunting guise

With 20mpen runes you have 20+3(15)+15+ possiblehauntingguise+15 = 95 mpen With 6% mastery + 35% voidstaff

= 95/41%

RIP my triboot velkoz I cant be lightningfasteteleportvelkoz anymore