Aren't we suppose to flag duplicate questions instead of posting comments about possible duplicates. Or it is the best practice to post a comment in addition to flagging a question as duplicate?
Best practice is to comment AND flag questions. Commenting helps drive people to the correct question, as well as gives the author a chance to try out the solutions on the duplicate. Often times questions are in fact a duplicate; however, occasionally it turns out that the question which was to be the duplicate doesn't actually solve the question. The resulting question then is edited to refine the issues at hand.