Abilities » Dual Armor | Energy » Shield Energy
Can I attach a Shield Energy to Lucario with the "Dual Armor" Ability?
It depends on when the Shield Energy is being attached. If Lucario already has a Metal Energy attached to it Dual Armor makes Lucario both Fighting & Metal types, so you CAN attach Shield Energy at that point. However, if Lucario does not already have a Metal Energy attached or if its Dual Armor ability has been shut off, then you CANNOT attach a Shield Energy to Lucario.
Source: TPCi Rules Team (2016-02-25)
Abilities » Dual Armor | Energy » Shield Energy
If Lucario with "Dual Armor" Ability has one Basic Metal and one Shield Energy attached, what happens if the Basic Metal Energy card is removed? Does Lucario remain a Metal type or would I have to discard the Shield Energy?
Once Lucario gains Metal typing due to Dual Armor it stays that way as long as a Metal Energy is attached. And Shield Energy provides a Metal Energy as long as it is attached to a Metal type Pokemon. So if only the Shield Energy remains, Lucario stays Metal type and the Shield Energy remains attached.
Source: TPCi Rules Team (2016-02-25)