It depends on what you mean by "Look, it's happening again".

Mass killings were not a new thing in the 20th century, they had existed in the past, and had actually occurred at least two times in the years leading up to the holocaust.

The Armenian Genocide, the mass, systematic, and racially based killings that actually coined the term "genocide" occurred earlier in the century.

Perhaps the best example of this occurred in 1937-1938. The Rape of Nanking was a product of the extreme racialization that the Japanese subjected the Chinese to. Depending on when you periodize World War II, you could make the argument that the Rape of Nanking actually occurred during World War II and a couple of years before the Holocaust. The Rape of Nanking was a byproduct of the Japanese proposed "Greater East Asia Co-Prosperity Sphere", which was similar in purpose and scope to Germany's "Generalplan Ost", the plan to exterminate and deport the slavic people of Eastern Europe and then "Germanise" the land. Like the German's, the Japanese wanted to turn a portion of the people native to their conquered land into slaves while exterminating millions of them. The Rape of Nanking is not officially classified as a "genocide", but it certainly fits all the of the necessary criteria.

So in short, mass killings had happened in the past, and two (one of them arguable) incidents of racially motivated genocide had occurred in the same century.

Also as a precursor to the Holocaust, Otto von Bismarck openly discussed the extermination of the Polish people. Some historians have made the argument that Hitler was really attempting to fulfill the vision of Bismarck by taking Eastern Europe and Germanising it.