A more thorough explanation:

Dual inheritance refers to the inheritance received by an adopted child through both their adopted family and their biological parent. This can become a problem if a relative of the biological parent adopts the child, as the child may qualify for two separate shares of inheritance.

If a child's mother dies and the maternal grandparents adopt the grandchild, and if a grandparent then dies without a will, the child may qualify for two separate shares of inheritance - one as a child and the other as a grandchild. In some places, the child is only allowed to inherit the adopted child's share, while in others, they take the larger of the two shares.