claude: warn when cell-renderings has duplicate names

When the `renderings` attribute contains duplicate names like
`[dark, light, dark, light]`, only the last cell output for each
name is used. This change adds a warning to alert users that
their earlier outputs are being silently dropped.

Fixes #13900

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gordonwoodhull

news/changelog-1.9.md

@@ -32,6 +32,7 @@ All changes included in 1.9:
 - ([#11929](https://github.com/quarto-dev/quarto-cli/issues/11929)): Import all `brand.typography.fonts` in CSS, whether or not fonts are referenced by typography elements.
 - ([#13413](https://github.com/quarto-dev/quarto-cli/issues/13413)): Fix uncentered play button in `video` shortcodes from cross-reference divs. (author: @bruvellu)
 - ([#13508](https://github.com/quarto-dev/quarto-cli/issues/13508)): Add `aria-label` support to `video` shortcode for improved accessibility.
+- ([#13900](https://github.com/quarto-dev/quarto-cli/issues/13900)): Warn when `renderings` cell option contains duplicate names. Previously, duplicate names like `[dark, light, dark, light]` would silently use only the last output for each name.
 
 ### `typst`
 

src/resources/filters/quarto-post/cell-renderings.lua

@@ -38,7 +38,12 @@ function choose_cell_renderings()
       end
 
       local outputs = {}
+      local seen = {}
       for i, r in ipairs(renderings) do
+        if seen[r] then
+          quarto.log.warning("duplicate rendering name '" .. r .. "' in renderings; only the last cell output with each name will be used")
+        end
+        seen[r] = true
         outputs[r] = cods[i]
       end
       local lightDiv = outputs['light']

tests/docs/smoke-all/2026/01/16/13900.qmd

@@ -0,0 +1,21 @@
+---
+title: "Test duplicate rendering names warning"
+format: html
+execute:
+  echo: false
+_quarto:
+  tests:
+    html:
+      printsMessage:
+        level: INFO
+        regex: "duplicate rendering name 'dark' in renderings"
+---
+
+```{r}
+#| renderings: [dark, light, dark, light]
+
+plot(1, main = "Plot 1 (dark)")
+plot(2, main = "Plot 2 (light)")
+plot(3, main = "Plot 3 (dark)")
+plot(4, main = "Plot 4 (light)")
+```