جَاءَ جَاءَ زَيْدٌ — Zayd certainly came.
First جَاءَ: past verb, with Zayd as its subject.
Second جَاءَ: repetition for verbal emphasis, fixed with fatḥah.
زَيْدٌ: nominative subject of the principal verb.
A past verb is indeclinable; do not assign it a nominal nominative case because it was repeated.