جَاءَنِي زَيْدٌ زَيْدٌ — Zayd—Zayd—came to me.
جَاءَ: past verb.
نِي: protective nūn plus first-person object pronoun, accusative in position.
First زَيْدٌ: nominative subject.
Second زَيْدٌ: verbal emphasis, nominative following the subject.
The second Zayd follows the first in case; it is not a second subject.