People need to apply duplicate affidavit certificate for two reasons. one , if you need that certificate before you are expecting it from that particular institution , for and example if you are a final year student and u need to apply for some job and the mark sheet is still awaiting , you can apply for provisional certificate and for that you need to put an application to the institution you are studying from . You should mention that you need it to apply for a job and as soon you get the original certificate you will submit the duplicate one.
One more reason is in case you lost any important certificate which means suppose you are carrying some valuable certificate and u lost it , the first thing u need to do is to provide an information regarding this incident to nearest police station . And if possible provide details of the original certificate. Because some of the certificates can be misused itt is always better to keep a photo copy of each and every document or certificate you have it will help u out to get the duplicate.
Once you file a first investigation report to police station, they will provide you with a receive copy of first investigation report. And with the help of this copy you can make a court affidavit and submit it to concern department of the related institution and you need to put a detailed application. Do not forget to mention as soon you get it that you will submit the duplicate to the concerned department.