What does the term "duck-quacking" mean?
Context:
Expect the casino industry to go all in on DFS as well, positioned as it is, with its savvy, resources and movement toward online casino gambling.
There can be little doubt of this duck-quacking. The business model of DFS totally mimics the pari-mutuel pool of racetracks and even in some ways, casino gambling.
Source: SeekingAlpha.com, about halfway into article.