This is a common request for examinations of mac images that include the active chat.db messages and the archived .ichat databases. This tip will also benefit examiners with Apple iCloud productions that have multiple snapshots of the sms.db.
1- In 'Communication' view, 'Messages' subview, select one of the duplicate messages, then select on the Metadata tab in the 'File Content Viewer' pane.
NOTE: The screen shots below display Conversation view, however you can do this in List View as well.
In the screen shot above the source file path for the chat.db file and the BlackLight file ID (BBT ID) for the file are marked in red.
In the screen shot above you will see the source file path of the duplicate message, the .ichat file, and the BBT ID for that file.
2- Next create a filter choosing 'BlackLight ID' equals <BBTID number>.
In the screen shot above the filter is 'BlackLight ID' equals '705085'
3- Click on the Apply button. The filter icon turns green and BlackLight displays just the messages from the chat.db source file.
4- You can tag the filtered results to generate a report on these messages or export them to CSV.
You can also invert the filter by choosing 'BlackLight ID' is not <BBTID number>.