you are right, but in practice radon isn’t used very often, only in iterative reconstruction.
frankly, we never thought about possibility to exploit this redundancy - angles, they are always angles.
with iradon, it is typical situation when at theta angle nearest half of the sample is in focus and the rest is out, at theta+180 they change places, and in order to get all volume more or less in focus, one needs to process both projections.
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