Translation:There is one person in the room.
this confused me too, but I think I understand it now, someone correct me if I'm wrong... ひとりis just a counter, ひとが determines what is being counted. The fact that they both have ひと in them is kind of misleading. The counter only has ひと in it because it happens to be derived from the same word as the subject being counted. But if you were to replace the person in the sentence with something else, like an apple, the distinction would be more apparent. Eg: へやに りんごが ひとつ あります。It becomes much more clear why the subject is required. Without りんごが the reader would not know what there is only one of.
In the specific circumstance of this question, ひとが is somewhat redundant because ひとり is a counter that is used exclusively for people, however for consistency sake it is left in. There is actually a word for this in English which is a "pleonasm", that is basically an unnecessary word that is left in the sentence for grammatical reasons. For example: In the sentence "It is raining" the pronoun "it" adds no meaning to the sentence because it's already understood what is raining because only the sky rains. However, to remove "it" from the sentence (Is raining) would sound awkward and would be incorrect because there must be a subject in every English sentence.
You're on the money here, but I don't think the 人が is redundant at all. Sure, 一人 can only count people, but what person is in the room alone? Without specifying, and without context, the subject is assumed to be "me", and the sentence becomes "I am in the room by myself".
Without the 一人 it's also not clear how many people are in the room. I feel like that would make it more like (though I'm taking a slight liberty here) "someone is in the room" rather than "a person is in the room" or "a person is in the room alone".
If if you translate literally, you could understand as "There is a person alone in the room". Hitori literally means one person, but in such contexts can mean alone, and futari can mean "two people together". I warn you it is fully contextual, such counter literally means one person, two, three...
It's the usual reading of 一人 when written left-to-right as it is here and it's a person's name. Hitori is actually written 1人. I wanted to write, "The person in the room is Kazuto." へやに人が１人います。I'd translate 「部屋に人が1人居ます。」 near as I can directly as, "The people in the room [perhaps looking only at this time] number one [in all]."