His plan was to find a bride for show then throw her away. He may be doing it in a straight forward manner and not adding a kid to the mix, but it's basically what his dad did to his mom.
In case people want to know, about the duke's father.
Spoilers from the novel, so if you proceed, you have been warned
He will appear in the second arc (second novel arc)
He's a shallow, irresponsible bastard. A very unpleasant character.
Is he the worst? No. Is he malicious towards the duke or Avril? Not really.
But he is an uncaring womanizer. He never cared about his domain nor the people within. He never did jack shit except having fun with his lovers. He never cared about the duke's mother either, and when she became unwell he just ignored her.
And also, he's pretty fucking dumb too.
I really dislike that entire arc solely because of him. He has ZERO redeeming qualities
The "necklace is quite expensive" is an excuse for "sorry, I'd love to assist but we can't do what you requested" (to bought necklace back or give him their name) because in next panel, the duke thinking "it's not working even with using the duke name? , must be people with certain status"
hence why the duke asking for previous seller name instead of buyer.
Holy cannoli. The Duke is actually smart enough to figure out about the necklace and its possible significance to Avril. Wonderful. I'm used to MLs who are more dense.