Note: You can have multiple Team-Building Exercise in play simultaneously. Unlike cards like Helicarrier or Avengers Mansion, it is not explicitly forbidden. It also lacks the unique symbol.
However, you can only apply one Team-Building Exercise per card played. This is because it specifies "play a card from your hand" instead of "reduce the resource cost".
For a character who uses multiple traits (e.g. Captain Marvel uses S.H.I.E.L.D, Avenger, Soldier and Aerial) this might be a useful tip.