@BillDubuque I am not sure whether this is what you're after, but it seems that both search and SEDE can recognize whether a post is closed as a duplicate.

There is a search option duplicate which you can combine with other search conditions. For example you can search for duplicate-closed posts in a given tag, combine this with date-range, etc.

According to Database schema documentation for the public data dump and SEDE in the PostHistory table you have If PostHistoryTypeId = 10, this field contains the CloseReasonId of the close reason which can be either 1 = Exact Duplicate or 101 = Duplicate.

Also in Postlinks table you have LinkTypeId 3 = Duplicate.

Probably you can find a few already existing queries related to duplicates.

As a side note, I have not received a ping from your message - but you probably knew that when you posted it, since you did not get my name was not offered in the autocomplete: Does ping work in chat with no autocompletion?

@MartinSleziak Yes, I have seen those options. I thought perhaps someone might already have written some tools to perform such trend analysis. Another item to push onto my todo list. I rarely use chat so I didn't know about the ping constraints. Thanks for the links.

@BillDubuque And what exactly do you mean by trend analysis?

Do you simply want percentage of questions in a given tag that are closed as duplicate?

Or percentage of questions in a given tag closed as duplicate per month/quarter etc.?

@MartinSleziak e.g. it'd be nice to see a graph over time of the percentage of questions closed as dupes in various tags.