Why is ‘for a second repeated in the paragraph beginning 'Conor frowned, and for a second the whole room seemed to get darker'? In A MONSTER CALLS

The repetition of "for a second" is done to emphasize how quickly Conor's sensations and emotions (which lasted for a second) were felt.

Explanation:

In "A monster calls" we are presented with the following paragraph:"Conor frowned, and for a second the whole room seemed to get darker, for a second it felt like the whole house was shaking, for a second it felt like he could reach down and tear the whole floor right out of the dark and loamy earth."

In this paragraph we can see the use of the figure of speech called Anaphor, which allows a word, or a group of words, to be repeated to emphasize some message that the author wishes to convey. In this paragraph we can see the repetition of "for a second", which was done to emphasize the speed with which Conor felt emotions and sensations.