what were "dual unionists" (US labor history)

I see in some historical texts that communists were charged with being “dual unionists” by other labor leaders in the 1920s and 1930s. What does this mean? Was this a label or strategy the communists themselves discussed or just an accusation thrown against them? What was the other union in “dual”? I see from googling aroudn a bit that it means scrapping existing unions to form a new big one. But is that all?

No, this is not an exam question. : )

Dual unions are competing unions. The old trade unions that concentrated on collective bargaining within the capitalist framework were not receptive to the goals of the Communists and Socialists, so initially, the parties set up alternative unions that were focused more on radical change, like the I.W.W. These unions were supposed to replace the trade unions, and revolutionary dogma encouraged dual unionists to “help” this process along.

Ultimately, Party leaders (including Lenin, back in the USSR) concluded that infiltrating and taking over trade unions would be more successful, but that didn’t go anywhere either.

Thanks.