This title has been subject of an unorthodox succession when the last duke Manfredo could leave the title not to his close relatives, but his distant cousin, Infanta Margarita Duchess of Soria.

The Infanta has two children: Alfonso, unmarried and w/o issue and Maria Zurita, who has a son born by FIV.

At Margarita's death, Alfonso (should he survive his mother) will inherit the Hernani title - the Soria having been created for life for Margarita.
If Alfonso won't marry and have legitimate issue, the H. title will pass to his sister.

Is her son considered legitimate for title succession purposes, being the child of a single mother, w/o knowing who his father was ?

This is a new reality who will probably spread in the future.
Has there been any other case among spanish aristocracy ?