海外is used to describe the students that they are from other countries.
According to baike(like wiki in china),现在泛指留居外国学习或研究的学生。“留学生”means the students who live abroad to study.
“留学生”and“海外”do have the same meaning in this sentence,but the two words put together don't make it redundant.
Instead,it's totally correct.And putting the two words together makes the sentence more accurate.
If there are any mistakes in my answer,please point them out.Learning English,too！
Well,suddenly I think I might be a little wrong.
This sentence could be redundant grammatically.But if you used it incautiously in your writting or a conversation,it'd be alright.The native speakers can totally understand you and probably they don't even notice this.
However,我们学校有很多留学生 is perfect.