When the Junk Arm trainer says "you can't choose Junk Arm", does it mean any Junk Arm already in the Discard Pile, or just this specific card?
It means you can't get any Junk Arm cards from the Discard Pile.
Source: Triumphant FAQ (2010-11-04), PUI Rules Team (2010-11-04)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_87