You should always link to those past submissions and remain otherwise neutral (or positive if you like). It is not and cannot be the submitters duty to closely track Hacker News for all stories that have been submitted in the past . Duplicates are inevitable, just writing “dupe” helps no one. Be constructive!
 I’m not entirely sure whether the submitter should check the front page before submitting. Since a bookmarklet is officially endorsed and prominently linked I would rather think that checking the front page is no requirement. You should maybe only be careful and check the front page or the new page when you want to submit a story about an event that you know will be a hot topic on Hacker News (say, a Google press conference).
And I agree that it is good practice at least to check at least the first page of Hacker News or the "news" section to avoid obvious duplicates .
All 4 current comments on this story are completely worthless.