Lilies are bulbs and can propagate from just one plant by growing bulbils, bulblets or offsets. Offsets grow near the parent in the ground and take a few years to flower. Leaves are normally the same colour but shorter. Depending on species the juvenile leaves can have a slightly different shape.
I have seen the same thing you have observed in purchased bulbs in the second season or after major trauma like a chipmunk digging up the parent. If the parent puts everything into flowering the next year the parent may not flower but it will have a lot of offsets.
No action is necessary, sit back and wait and you will have more lilies.