When holding one item with something like +10 health or any other attribute in the main hand, and then putting something in your armor slot with the same attribute (health, speed, etc.) only one of the two will function. If you first put on the armor and then held the main hand item, only the item will work and vice versa. I am not sure if it happens when the attributes are different types or if happens when the two items go in your offhand and mainhand for the attribute to be applied. I have also not tested this in 1.8. It may not be intended that attributes are stacked up in which case this is not a bug, just a feature.