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I feel that science is the fallback point which the grinding community relies on to provide a natural immunity to fascism. Scientific progress of the kind that we are engaging in is an inherently collectivist endeavor. Any out-group that we encounter is invited to join us in our communal progress. If someone does not identify with Transhumanism, it's likely they'll still indirectly benefit from the fruits of our labors.
BirdMachine and Cassox have the right of it. Peer review is a reliable way to spot a supremacist, eugenicist, or a fascist. Follow that up with some public shaming when their assertions prove false, and a significant portion of the problem may be taken care of.