Double genocide theory is a form of holocaust denial.

Remember when Communism was allied to the Nazis and invaded Poland with them

Not to mind this is just straight up bullshit. Non aggression pacts are not the same as allying with the Nazis. The non aggression pact was necessary to delay the inevitable long enough to industrialise and build up a force to fight the Nazis largely alone as the western powers had continuously refused to form an anti nazi pact since 1933. The soviets were also the last major power to sign a non aggression pact with the Nazis. The USSR broke the nazi beast took the majority of the casualties and killed the majority of the Nazis.

The USSR spent years trying to build an anti-Nazi alliance. 1933 they proposed collective security at the League of Nations. 1935 they signed mutual defense pacts with France and Czechoslovakia. Spring 1939 they sat in Moscow for months begging Britain and France for a real triple alliance. The West stalled. Refused to guarantee the Baltics. Refused to let the Red Army cross Poland to actually fight Hitler. Poland’s elite, more scared of workers than of Nazis, said no too and instead joined Hitler in attacking Czechoslovakia. The Molotov-Ribbentrop Pact happened because liberals handed Hitler Eastern Europe rather than work with socialists.

When Soviet troops entered eastern Poland September 17 1939, the Polish state had already collapsed. Government fled to Romania September 15. Warsaw was burning. The army was broken. The lands the USSR moved into? Not Poland proper. Territories Poland had seized and occupied by force in 1919-1921 from Belarus, Ukraine, and Lithuania. Moving in to secure this land also saved millions from extermination.