think后面的it是形式宾语,our duty是宾语补语,our duty后面的to study well是真正的宾语,这种结构在英语里很多见,作真正宾语的,可以是动词不定式,动名词或从句,比如说：
The book will help me to study English,and it makes it easy to study English.
I consider it important that we should learn English well.
He found it no use telling her the truth.
希望我能帮助你解疑释惑.