One-ness is the state of "Consciousness".
Consciousness is one. But awareness is varies creature to creature.
One-ness and duality exist simultaneously.
Duality is a yin-yang state. Together they exist.
We are part and parcels of the whole.
The original Supreme Personality of Godhead is one person.
The creation is multifaceted.
The original Supreme Personality of Godhead is He who is infinitely full in all opulences ("Bhagavan"), specifically:
God is the original person who possesses all Beauty, Fame, Intelligence, Power, Wealth, and Renunciation, the first person, with his own eternal and transcendental name, fame, form, personality, paraphernalia, entourage and pastime, whose body is the absolute form of 'Sat-Chit-Ananda'—Eternity. Cognizance, and Bliss.
We souls are individual sparks of the whole.