This is very simple, the Duke (& king) found out about her being a fake immediately at the weeding which was held in the capital. It was clear it was the capital from the following circumstantial evidence:
1. The crowded church and the fact that the emperor was present (the Emperor especially of what we have seen of him till now is in no way the type of person to travel North for the weeding, and even those with better personalities wouldn't do it, such events are mostly held in the capital anyway).
2. How much light and physical warmth was portrayed within the church, all people where dressed lightly especially the bride, you can't do that in the North you will freeze to death.
You had seen the discussion the duke had with the emperor to leave it be, and within the mansion from the start of the manhwa the Duke never made a single mention of him knowing this, the general direction hinted is that after the discussion with the Emperor which was in the capital he never let anyone know that she wasn't the real count's daughter, remember not even his close knight knew anything, and the captured maid only started spilling the beans once in prison and as far as we know there is no prison golem, just an inner castle golem and and wall/outer golem. Unless later on is shown that the inner golem can also manifest in prison or there is a prison golem there is no plot hole here.
PS: yeah.... i joined just to reply to this one year old post lol.