Hi Simma
I have a similar situation, when checking on certs for the second marriage I found the lady in question and it was a bigamous marriage, her first husband was still alive, I purchase both certs so I know a marriage took place on both occasions.
I also have a couple who married in London after living in Essex, I discovered it was an illegal marriage, he had married his late wife's sister, it wasn't found out till they returned to Essex to have a child baptized, the incumbent wrote on the baptism record that this marriage was illegal, they had obviously run away to get married but were found out on their return..
Single surnames on re-marriage, I have a relative who married just 2 years ago having been divorced, she used her single name on remarriage, I didn't say a word for fear of putting a spanner in the works as it were but I would have thought she would have had to use her married name. Scotland is something a little different, even on a gravestone they use their single surname
Happy hunting
Louisa Maud