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/dev/requirements.md

@@ -179,6 +179,17 @@ card into the library. Both are needed.
 original filename) and by content hash, offering skip or import-as-new. Camera filenames wrap at
 `IMG_9999`, so filename alone is insufficient. Existing catalog duplicates are detectable on demand.
 
+**FR-CAT-11a — Consolidating catalog duplicates.** Files already in the library more than once
+(same root, camera, capture time and size) shall be listed on demand as groups, from the library
+and from Settings. Before anything moves, each group shall be proved the same file by a stored full
+digest or by a digest of the first and last megabyte of every copy, and its copies' develop edits
+compared; a group that differs in either is left out and the review says why. One copy survives —
+not under a backup-looking folder, then camera-named, then oldest, overridable per group — and the
+others' collections, keywords, highest rating, agreed flag and label, and faces are merged onto it,
+with disagreements reported rather than decided silently. Each group is merged and its other copies
+moved to the trash (FR-CAT-15) in one transaction: wholly consolidated or untouched. Nothing is
+deleted.
+
 **FR-CAT-12 — Versions (virtual copies).** An image may carry multiple named `Version`s, each with
 an independent edit graph, without duplicating source data. Versions are creatable, nameable,
 deletable, and independently exportable; one is the default.

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;