Tankies don't sweat doublethink. Whichever answer is more convenient at the moment, even if it contradicts their immediately previous statement, is correct.
Also sprinkle in a dash of inconsistent definitions. They're talking about end-stage communism when they say communism has never been tried, except when they're talking about socialist states on the path to communism, of course. In which case communism has been tried and is working, unless you succeed in bringing up some part of their favorite hypercapitalist state that's too capitalist to ignore, in which case the definition swaps back.
Simple!