articles - Using "the" or "a" for an item already mentioned

I was reading a topic article in a English grammar book.

In an explanation, the following guideline was mentioned:

Use the to refer something that has already been mentioned.

While I was doing an exercise, I faced the following sentence.

3. Our teacher gave us ______ test today. It was ______ really hard test.

There were questions on there which I didn't even understand. But based on the explanation above, the answer in the first sentence should be a, and the answer in the second sentence should be the.

But when I saw the answer, there was a in first sentence and a in the second sentence also.

How is this possible? Based on the explanation, the answer in the second sentence should be the! Please explain.