It has a flat for a reason
If you have more than one screw, but only one flat surface on the shaft then only tighten the one set screw.
Refrain from overtightening set screws, they need to bite into the shaft but you don’t need to mangle the poor thing. Both setting on the curve and overtightening of these conditions can make it hard or impossible to remove the blade or wheel later. Simple, but important.
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade