To understand this question better you would benefit from reading the book “Guidance for the Seeker of TRUTH” or “The Way Back to TRUTH“. Duality can be thought of as everything manifest, which includes all the spiritual dimensions. Unity is that which is one, the Source, God, call it what you will.
The “spiritual journey” as the name suggests is the journey your spirit takes to get back to Unity, a journey that starts in Duality, down in the physical realms.
So this is an interesting question because it asks what the potential of the soul is. Keeping things very simple, the soul and spirit are two different parts of you and they both span many lifetimes, so that this life, is just one of many that your soul (and spirit) will experience.
As we discuss in the various books, videos and articles on this site your soul is also on a journey and the simplest understanding of this journey is that your soul is trying to become more loving, to embody and demonstrate the qualities of the highest love which is unconditional love. Unconditional love emanates from the Source and can be considered the start of Duality, although it is more accurate to say that it is by Grace that it comes into Duality.
Since your soul is a vehicle of experience then it always needs duality, in order to have experience, learning through contrast and so your soul’s journey can take it to the very edge of Duality but not into Unity. This movement is achieved through meditation on Light and Sound energy and your soul reaches its zenith at the moment of enlightenment.