From b5c7eddf8fc1447df8b7fa315ef4152ddfc1a6dc Mon Sep 17 00:00:00 2001
From: Michael Anderson <andermic@gmail.com>
Date: Thu, 15 Oct 2026 17:24:05 -0500
Subject: [PATCH] Photo Merge: Merge a photo named twice only once (#1947)

---
 crates/engine/src/merge.rs       |  6 ++++--
 crates/engine/src/tests_merge.rs | 24 ++++++++++++++++++++++++
 docs/whats-new.md                |  7 +++++++
 3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/crates/engine/src/merge.rs b/crates/engine/src/merge.rs
index 26c69c63d..76bcfd7f2 100644
--- a/crates/engine/src/merge.rs
+++ b/crates/engine/src/merge.rs
@@ -404,10 +404,12 @@ pub fn parse(id: &str, p: &Value) -> Result<(MergeKind, MergeFinish)> {
 }
 
 impl Session {
-    /// Plan a merge of `ids` (catalog order of the given ids is kept).
+    /// Plan a merge of `ids` (catalog order of the given ids is kept). A photo named more than once is merged once
+    /// (issue #1947): it adds nothing to the merge, and each copy would be decoded and held in memory.
     pub fn plan_merge(&self, kind: MergeKind, finish: MergeFinish, ids: &[PhotoId], preview: bool) -> Result<MergeJob> {
         let mut sources = Vec::new();
-        for id in ids {
+        let mut seen = std::collections::HashSet::new();
+        for id in ids.iter().filter(|id| seen.insert(**id)) {
             let p = self.catalog.photo(*id).ok_or(lightcraft_catalog::CatalogError::NoPhoto(*id))?;
             match &p.source {
                 Source::File { path } => sources.push((*id, path.clone())),
diff --git a/crates/engine/src/tests_merge.rs b/crates/engine/src/tests_merge.rs
index aaa59acb1..ed524ae3d 100644
--- a/crates/engine/src/tests_merge.rs
+++ b/crates/engine/src/tests_merge.rs
@@ -226,3 +226,27 @@ fn panorama_size_is_held_to_memory() {
     assert_eq!(mp(fit_in_memory(&asked, photos, gb(16))), 20.0, "a size asked for is kept when it fits");
     assert_eq!(fit_in_memory(&full, 1e6, None), fit_in_memory(&full, 1e6, gb(8)), "memory unknown: taken as 8 GB");
 }
+
+/// A photo named twice is merged once (issue #1947): `[a, a, a]` is one photo, too few to merge, and `[a, b, a, b]`
+/// merges two frames rather than holding four in memory.
+#[test]
+fn a_photo_named_twice_is_merged_once() {
+    let dir = temp_dir("dup-ids");
+    let mut paths = Vec::new();
+    for (i, b) in lightcraft_merge::synth::bracket_dngs(96, 64, &[-1.0, 1.0]).unwrap().into_iter().enumerate() {
+        let p = dir.join(format!("IMG_{i}.dng"));
+        std::fs::write(&p, b).unwrap();
+        paths.push(p.to_string_lossy().to_string());
+    }
+    let mut s = Session::new().with_fs();
+    let ids = import(&mut s, paths);
+    let (a, b) = (ids[0], ids[1]);
+    for cmd in ["merge.hdr", "merge.panorama"] {
+        let e = s.execute(cmd, &json!({"ids": [a, a, a], "preview": true})).unwrap_err();
+        assert!(e.to_string().contains("at least 2"), "{cmd}: {e}");
+    }
+    let (kind, finish) = crate::merge::parse("merge.hdr", &json!({})).unwrap();
+    let job = s.plan_merge(kind, finish, &[a, b, a, b, a].map(lightcraft_catalog::PhotoId), true).unwrap();
+    assert_eq!(job.sources.iter().map(|(p, _)| p.0).collect::<Vec<_>>(), vec![a, b]);
+    let _ = std::fs::remove_dir_all(&dir);
+}
diff --git a/docs/whats-new.md b/docs/whats-new.md
index 97e1367a8..9819ae670 100644
--- a/docs/whats-new.md
+++ b/docs/whats-new.md
@@ -1,5 +1,12 @@
 # What's new in LightCraft
 
+## 2026-10-15
+
+### Photo Merge takes each photo once
+- A merge that names the same photo more than once (through `ids` on the control channel, MCP or the CLI) now merges
+  it once (issue #1947). Naming one photo three times no longer "merges" it with itself, and a long repeated list no
+  longer decodes the photo again for every entry.
+
 ## 2026-10-14
 
 ### Lightroom radial masks on the right side