I run two job boards. Job Board A accepts unique posts as well as mirrors several client job boards. Job Board B is about to be published and has a larger scope so mirrors all of Job Board A plus accepts unique posts.
Should Job Board A use canonical tags for job postings mirrored from the client job boards? I haven't since 2008 and my search results are decent.
Should Job Board B use canonical tags for job postings mirrored from Job Board A?
Job Board B should use canonical tags for job postings mirrored from Job Board A like, In simple add the code in Job board B page and mention the repective link of job board B.
Please note that by doing this your new pages on Job Board B wont rank but atleast you will have chance to get traffic from other sources like social, referrals and direct traffic. If you chose not to add canonical tag: The impact will be double, The pages on Job Board B wont rank as well as your site will open up to a Google Panda penalty for content duplication.