Describe consolidating duplicate originals in the manual and the register

FR-CAT-11a records what #67 built: groups of the same file listed on
demand, proved the same before anything moves, merged onto one survivor
and the rest trashed a group at a time. The manual's new section under
The library says how to reach the page, how the copy that stays is
chosen, what the check reads and what merges.

docs/manual/README.md

@@ -106,6 +106,37 @@ to have it show another, open it and click the ring on the frame you want.
 With faces indexed, narrowing the grid to a person puts `Eyes open` beside
 their name on the filter bar, which leaves out the frames where they blinked.
 
+### Duplicate originals
+
+A library put together by hand often holds the same RAW more than once — a
+dated folder, a `bck` folder beside it, a renamed copy another program
+exported. When the catalog finds files with the same camera, capture time
+and size in more than one place, `Duplicate originals` appears under the
+trash in the sidebar with how many there are; Settings offers the same page
+beside the other whole-library passes.
+
+The page lists each group with its picture and its paths. One copy is
+marked `Stays`: the one outside a folder named like a backup (`bck`,
+`backup`, `copy`, `old`…), then the one still named the way the camera named
+it (`_MG_4623`, `IMG_0001`, `DSC_0042`), then the one catalogued first. Tap
+another path to keep that copy instead, and untick `Include` to leave a
+group alone.
+
+Nothing moves until `Check` has read the first and last megabyte of every
+copy and each copy's sidecar. A group whose files differ, or whose copies
+carry two different edits, is marked skipped and says why — two edits of
+one frame are kept for virtual copies. What the check reads is kept, so a
+second visit costs nothing. The line under each group says what the copy
+that stays will gain: the highest rating, every keyword and collection, a
+flag or label the copies agree on, faces and the names on them, and the
+edit if only a copy had one. Where the copies disagree on a flag or a
+label, the one that stays keeps its own and the line says so.
+
+`Move N copies to trash` does it, one group at a time: each group is merged
+and its spare copies moved together, or not at all. The copies go to the
+trash, not away — select them in the trash view and `Restore` puts them
+back where they were.
+
 ## Developing a photograph
 
 Click a thumbnail to open it. The column on the right is every adjustment;