hugolib: Fix page/section name collision regression

Fixes #15046

When a regular page (e.g. content/s1.md) and a section (content/s1/)
share the same tree key, the assembler must not overwrite the real page
with a synthetic section. Restore the existence guard that was present
in v0.152.2.

The fix checks if a node already exists at the section key before
inserting a synthetic section during root section creation.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

hugolib/content_map_page_assembler.go

@@ -508,6 +508,15 @@ func (a *allPagesAssembler) doCreatePages(prefix string, depth int) error {
 				// Try to preserve the original casing if possible.
 				sectionUnnormalized := p.Unnormalized().Section()
 				rootSectionPath := a.h.Conf.PathParser().Parse(files.ComponentFolderContent, "/"+sectionUnnormalized+"/_index.md")
+
+				// A regular page (e.g. content/s1.md) may share the same tree
+				// key as its section (both map to /s1). In that case the page
+				// itself already occupies the section root; creating a synthetic
+				// section on top of it would overwrite the real content.
+				if _, exists := treePages.GetRaw(rootSectionPath.Base()); exists {
+					return true, nil
+				}
+
 				var rootSectionPages contentNode
 				rootSectionPages, _, err = transformPages(rootSectionPath.Base(), &pageMetaSource{
 					pathInfo:        rootSectionPath,

hugolib/site_sections_test.go

@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -148,3 +149,36 @@ All: {{ .Title }}|{{ .Kind }}|
 	b.AssertFileExists("public/a/b/index.html", false)
 	b.AssertFileContent("public/a/b/c/index.html", "All: C|section|")
 }
+
+// See issue 15046.
+func TestSectionAndSiblingPageURLs(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['home','rss','sitemap','taxonomy','term']
+uglyURLs = false
+-- content/s1/p1.md --
+---
+title: content/s1/p1.md
+---
+-- content/s1.md --
+---
+title: content/s1.md
+---
+-- layouts/all.html --
+{{ .Title }}
+`
+
+	b := Test(t, files)
+
+	b.AssertFileContent("public/s1/index.html", "content/s1.md")
+	b.AssertFileContent("public/s1/p1/index.html", "content/s1/p1.md")
+
+	files = strings.ReplaceAll(files, "uglyURLs = false", "uglyURLs = true")
+
+	b = Test(t, files)
+
+	b.AssertFileContent("public/s1.html", "content/s1.md")
+	b.AssertFileContent("public/s1/p1.html", "content/s1/p1.md")
+}