This came up in a game recently, where the Nurgle player tried to use this card twice in a region. None of us could really say whether or not that would allow placing a corruption in adjacent regions for each card, or just one because the wording mentions dominating the zone, which you can technically only do once.

So would two cards in the same region cause double the corruption, or would two cards be wasting one as it could only affect a given zone once?

Two Creeping Deaths will give two corruption. The card reads "If you dominate this region, place one corruption token in each adjacent region." No limitation is stated on the card, so if you dominate a region with two Creeping Deaths the effects of both cards do take place.

Double CD is one of Nurgle's strongest moves in the HR expansion, because of the potential of a lot of corruption being placed AND possibly several DAC. Try not to let the Nurgle player get a chance to do this...

Agreed. It can be assumed that two of any card played in the same region will double the effect.