fix(content): helpful message for DuplicateContentEntry (#9492)

* helpful message for DuplicateContentEntry

* add changeset

3 files changed, 17 insertions, 3 deletions

diff --git a/.changeset/gold-zebras-burn.md b/.changeset/gold-zebras-burn.md
new file mode 100644
index 000000000..8cb15a42b
--- /dev/null
+++ b/.changeset/gold-zebras-burn.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Improves error message for the case where two similarly named files result in the same content entry.
diff --git a/packages/astro/src/content/vite-plugin-content-virtual-mod.ts b/packages/astro/src/content/vite-plugin-content-virtual-mod.ts
index 92d47003e..7195ffffe 100644
--- a/packages/astro/src/content/vite-plugin-content-virtual-mod.ts
+++ b/packages/astro/src/content/vite-plugin-content-virtual-mod.ts
@@ -261,7 +261,12 @@ export async function generateLookupMap({
 					if (lookupMap[collection]?.entries?.[slug]) {
 						throw new AstroError({
 							...AstroErrorData.DuplicateContentEntrySlugError,
-							message: AstroErrorData.DuplicateContentEntrySlugError.message(collection, slug),
+							message: AstroErrorData.DuplicateContentEntrySlugError.message(
+								collection,
+								slug,
+								lookupMap[collection]!.entries[slug],
+								rootRelativePath(root, filePath),
+							),
 							hint:
 								slug !== generatedSlug
 									? `Check the \`slug\` frontmatter property in **${id}**.`
diff --git a/packages/astro/src/core/errors/errors-data.ts b/packages/astro/src/core/errors/errors-data.ts
index 86ae6bbb5..9e767e659 100644
--- a/packages/astro/src/core/errors/errors-data.ts
+++ b/packages/astro/src/core/errors/errors-data.ts
@@ -1292,8 +1292,12 @@ export const DataCollectionEntryParseError = {
 export const DuplicateContentEntrySlugError = {
 	name: 'DuplicateContentEntrySlugError',
 	title: 'Duplicate content entry slug.',
-	message: (collection: string, slug: string) => {
-		return `**${collection}** contains multiple entries with the same slug: \`${slug}\`. Slugs must be unique.`;
+	message: (collection: string, slug: string, preExisting: string, alsoFound: string) => {
+		return `**${collection}** contains multiple entries with the same slug: \`${slug}\`. ` +
+			`Slugs must be unique.\n\n` +
+			`Entries: \n` +
+			`- ${preExisting}\n` +
+			`- ${alsoFound}`;
 	},
 } satisfies ErrorData;