It's always best to run the same tires on all 4 corners (make/model, not sizes).
But, at the very least run identical tires on the same axle (seems fairly evident, but you'd be surprised at the number who don't).
Mixing tires may have some unforseen changes in the handling, braking and feel of the car. These would be most accentuated on the track, ax, high speed, as well as in the wet. But, usually, it's not at an unnacceptable level.
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