More specifically, I saw a question in French written like this: "Qu'arrive-t-il à Lucas". Why is the "il" necessary; isn't this a repetition of the "que" (therefore making it redundant) and therefore this question reads "What arrives it at Lucas"? I simply don't see why they added that "il".
In your example, the
que) points to whatever is happening to Lucas, and the
il is impersonal. Like in the statement
It here isn't really something or someone.
So just as you could say and ask the followings :
Does it rain?
You can say and ask :
Il arrive quelque chose à Lucas.
Qu'arrive-il à Lucas?
Careful though! In other situations,
il in a question would be a personal pronoun. Like in :
Lucas est à l'école.
And in that case
Lucas and it would be redundant to ask :
Où est-il, Lucas?
To conclude, some sentences use impersonal pronouns, and if you flip those sentences into questions, you will find again the impersonal pronoun. The more you read and speak French, the more you will be able to recognise them! For a start, here are a few examples :
Il arrive que..
Il pleut, Il neige, Il vente...
Il est tard, Il est tôt...