'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??
=)