When you expressed one's action, you can use "を". When you expressed one's behavior, you can use "が". Below are the examples. わたしは　まほうを　べんきょうした。 わたしは　まほうが　とくいだ。 わたしは　おさけを　のんだ。 わたしは　おさけが　すきだ。 わたしは　ほんを　よんだ。 わたしは　ほんが　ほしい。

'Double は in 「わたしは　まほうは　ひっしに　べんきょうした」' indicates that you had more options that you might have studied so hard. Had you in this situation, You had to study so hard, not only 'まほう’, but also housework, homework or any other things. But only 'まほう’ was object that you wanted to engage. Therefore, if you say 'わたしは　まほうは　ひっしに　べんきょうした to others. Others might wonder ' I see, but how was about other things you might have to do?'. Does it make a sense to you?? =)

わたしは　まほうは　ひっしに　べんきょうした Meaning of the words is "I studied only the magic very hard". When you expressed one's only action, you can use "は". I replied it once again, because it was settled in comment.