Enough people were wanting to know (myself included) so I risked a couple plat on a test:
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As you can see from these pics, putting a rune on a head piece that already has the same bonus as the head piece WILL stack! This is good news for the experimentor/change up artist. One can have different head peices for different attributes and just change one peice of armor to switch which attribute get's TWO points of bonus (or more if you want to sacrifice HP *shudder*). This would be particularly useful to warriors who could have vigor, absorption, strength, and tactics on their armor and switch between sword/axe/hammer on the head.
PS: for the observant/curious who might be wondering, I often go necro secondary when I solo, but am more often monk secondary when grouping (even with henchmen).
C



