From the SRD on damage reduction -
If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.
Now i understand why you may not want multiple magic item(s) DR to stack, but in the instance of were you have a Feat and Magic Item combination i’m not sure that makes sense.
Adamantine (Heavy Armor) provides a 3/- it could stack with a Adamantine shield or potentially a feat. Like Armor specialization, or the Roll With It feat from Savage Species.