Yes, that was the standard procedure.
It's easy to tell which is which.
The situation described by ADP results in two index listings with different references.
If the references in the index are the same it's one record indexed twice (or more), whether it's a birth, marriage or death. This can refer to someone who is illegitimate, or someone who is legitimate but known by a stepfather's surname, or a woman who has been married more than once.