If you are using GUI, make an itemtype array and put all damage items in it, and one with all armour in it. When a unit picks up an item, check if the gained item is in one of those arrays, and if it is, check if the unit has another item type that appears in that array. Then, remove the item from the unit somehow.
If you are using JASS, there are thousands of possible answers. Check the resource section, I believe you would find something useful there. | 
