The concealed subject pronoun of «qāma» points back to Zayd. First, Zayd is introduced as the principal subject; within the predicate clause, the verb's subject also identifies Zayd.
كَمَا فِي «زَيْدٌ قَامَ» صَرْفُهُ ذَلِكَ الضَّمِيرَ إِلَى الْمُبْتَدَإِ ثَانِيًا فَيُكْسَى الْحُكْمُ قُوَّةً
In «Zayd qāma», the pronoun's reference back to the subject a second time gives the judgment added force.
This does not mean two independent factual claims are made. Rather, two grammatical relationships converge upon the same referent.