I've always wondered about this. If both players have the same stadiums can they bump each others stadiums (player A bumping player B then player B bumping player A) or since they are the both the same stadiums you can't play it to bump it.
On the bottom of every Stadium, the text says "If another card with the same name is in play, you can't play this card." Meaning, no Stadiums can not be "bumped" by another Stadium of the same name.
It is a meta rule that you cannot play a Stadium card if one with the same name is in play, even if it's your own card in play. Stadium cards now actually include this text on the card (it's been this way for a while).