Many thanks, theducks and capink, very clever ideas ! However only for the case of identical titles, authors and languages, if I understood you.
First I followed theducks process :
I found these duplicates (same titles, authors) and corrected their language field.
I adjusted the merging options for copying to another library.
I adjusted this selectes list of books (using a special column) to manage the case of different formats (PDF vs EPUB)
I created a temporary library, and moved these selected books there, then back to my main library.
This allowed to delete 70 rows within a list of 140 duplicates, in a few minutes : thank you for that !
And I also used your idea to look at the pages count.
But now 1000 duplicates with similar titles and similar author still remain... And most are real duplicates.
In many cases, the author is spelled lightly differently between two duplicates. I know that the plugin can address that, but it seems a very long process.
Then I tried capink method. Through Options/Advanced I associated the CTL+K shortcut with Add to "DUP" list in the Reading List plugin.
This allows to keep trace of the 1000 Find Duplicates results after quitting the Find Duplicates plugin (another way would be to write in some custom column).
But then I am more or less stuck. I can manually delete them, but I have 1000 rows.
So I still guess it would very very useful to be able to select all rows but the first one in each duplicates group.
Or if this is not possible for technical reasons, maybe it is possible to export and reimport the list to excel ? Then I would edit it within excel.
I will investigate further, but not tonight
Thanks again