The only cross-branded Dubl Duck I know if is the mysterious and rare Puma-Duck, of which I know of only two examples, and one of them is in my shaving cabinet
But yeah, if it's a Case blade in Duck scales then that's pretty certainly all it is. Case razors were pretty good though, so if it's in good shape it's probably worth acquiring if the price is reasonable.
The key is what does the blade say. You can't go by the scales. The Puma-Ducks say Puma on one side and either Bresnick or Bresduck on the other so you know its genuine. Actually there are a few other strange ducks out there. I've seen another made by a french maker with Bresnick on one side and the maker on the other. I think for awhile Bresnick had several makers for his razors.