I just read comments of brother ashar and I disagree with second and third part. It's always better to prepare whole syllabus., specially for General science and ability.
Many times it happens that an examiner repeats questions for consecutive years.
In 2014 Punjabi paper , there were two questions asked which were asked before in 2013. One of my friends, well, he left those topics and then it was quite difficult for him to handle that paper. He passed but got low marks.
Main thing is this that if there is even 2 percent of chance of questions getting repeated again,as said by Ashar ,still we must not leave any topic. Becasue no one knows that if this 2 percent chance comes turns into reality ,then maybe you will be in some trouble. And you will only realize this thing in examination room. Not before that.
It happens sometimes that they surprise you with same questions that were asked in past year. And sometimes ,well mainly this thing is for PMS exams ,that nearly whole paper of past immediate year repeats. And same can happen in CSS exams too.
You can find repeated questions of EDS- that is science part of the paper ,in general science and ability section of this forum.
And it's good if you prepare those topics related to current issues as said in above post.and there is a chance that they will be asked. Basically ,it's a smart work . But still it's not a definite thing that these topics will be asked.Anyone can't be hundred percent sure. So ,still it's better to prepare whole syllabus.

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''Sufficient for us is Allah , and He is the best Disposer of affairs" Al-Quran