if i repeqt piercing light, it can be defy'd?

Yes — Defy can counter a repeated Piercing Light.

Defy checks the printed cost (2E/1P), not the total paid after Repeat, so it's a valid target. If it resolves, the whole spell is countered — both instances — and the Repeat cost is not refunded (FAQ #9464, Rule 425.1.c).