diff --git a/Sources/Publish/Internal/HTMLGenerator.swift b/Sources/Publish/Internal/HTMLGenerator.swift
index 411bf428..8683c722 100644
--- a/Sources/Publish/Internal/HTMLGenerator.swift
+++ b/Sources/Publish/Internal/HTMLGenerator.swift
@@ -15,32 +15,32 @@ internal struct HTMLGenerator<Site: Website> {
     let context: PublishingContext<Site>
 
     func generate() async throws {
-        try await withThrowingTaskGroup(of: Void.self) { group in
-            group.addTask { try await copyThemeResources() }
-            group.addTask { try generateIndexHTML() }
-            group.addTask { try await generateSectionHTML() }
-            group.addTask { try await generatePageHTML() }
-            group.addTask { try await generateTagHTMLIfNeeded() }
-
-            // Throw any errors generated by the above set of operations:
-            for try await _ in group {}
+        try await withThrowingTaskGroup(of: (substep: String, paths: [Path]).self) { group in
+            group.addTask { (substep: "Copy theme resources", paths: try await copyThemeResources()) }
+            group.addTask { (substep: "Generate index", paths: try generateIndexHTML()) }
+            group.addTask { (substep: "Generate sections", paths: try await generateSectionHTML()) }
+            group.addTask { (substep: "Generate pages", paths: try await generatePageHTML()) }
+            group.addTask { (substep: "Generate tags", paths: try await generateTagHTMLIfNeeded()) }
+
+            try await validate(group)
         }
     }
 }
 
 private extension HTMLGenerator {
-    func copyThemeResources() async throws {
+    func copyThemeResources() async throws -> [Path] {
         guard !theme.resourcePaths.isEmpty else {
-            return
+            return []
         }
 
         let creationFile = try File(path: theme.creationPath.string)
         let packageFolder = try creationFile.resolveSwiftPackageFolder()
 
-        try await theme.resourcePaths.concurrentForEach { path in
+        return try await theme.resourcePaths.concurrentMap { path -> Path in
             do {
                 let file = try packageFolder.file(at: path.string)
                 try context.copyFileToOutput(file, targetFolderPath: nil)
+                return path
             } catch {
                 throw PublishingError(
                     path: path,
@@ -51,22 +51,27 @@ private extension HTMLGenerator {
         }
     }
 
-    func generateIndexHTML() throws {
+    func generateIndexHTML() throws -> [Path] {
         let html = try theme.makeIndexHTML(context.index, context)
-        let indexFile = try context.createOutputFile(at: "index.html")
+        let path = Path("index.html")
+        let indexFile = try context.createOutputFile(at: path)
         try indexFile.write(html.render(indentedBy: indentation))
+        return [path]
     }
 
-    func generateSectionHTML() async throws {
-        try await context.sections.concurrentForEach { section in
-            try outputHTML(
+    func generateSectionHTML() async throws -> [Path] {
+        try await context.sections.concurrentFlatMap { section -> [Path] in
+            var allPaths = [Path]()
+
+            let sectionPath = try outputHTML(
                 for: section,
                 indentedBy: indentation,
                 using: theme.makeSectionHTML,
                 fileMode: .foldersAndIndexFiles
             )
-            
-            try await section.items.concurrentForEach { item in
+            allPaths.append(sectionPath)
+
+            let sectionItemPaths = try await section.items.concurrentMap { item -> Path in
                 try outputHTML(
                     for: item,
                     indentedBy: indentation,
@@ -74,11 +79,14 @@ private extension HTMLGenerator {
                     fileMode: fileMode
                 )
             }
+
+            allPaths.append(contentsOf: sectionItemPaths)
+            return allPaths
         }
     }
 
-    func generatePageHTML() async throws {
-        try await context.pages.values.concurrentForEach { page in
+    func generatePageHTML() async throws -> [Path] {
+        try await context.pages.values.concurrentMap { page -> Path in
             try outputHTML(
                 for: page,
                 indentedBy: indentation,
@@ -88,9 +96,9 @@ private extension HTMLGenerator {
         }
     }
 
-    func generateTagHTMLIfNeeded() async throws {
+    func generateTagHTMLIfNeeded() async throws -> [Path] {
         guard let config = context.site.tagHTMLConfig else {
-            return
+            return []
         }
 
         let listPage = TagListPage(
@@ -99,13 +107,15 @@ private extension HTMLGenerator {
             content: config.listContent ?? .init()
         )
 
+        var allPaths = [Path]()
         if let listHTML = try theme.makeTagListHTML(listPage, context) {
             let listPath = Path("\(config.basePath)/index.html")
             let listFile = try context.createOutputFile(at: listPath)
             try listFile.write(listHTML.render(indentedBy: indentation))
+            allPaths.append(listPath)
         }
 
-        try await context.allTags.concurrentForEach { tag in
+        let tagPaths: [Path] = try await context.allTags.concurrentCompactMap { tag -> Path? in
             let detailsPath = context.site.path(for: tag)
             let detailsContent = config.detailsContentResolver(tag)
 
@@ -116,16 +126,35 @@ private extension HTMLGenerator {
             )
 
             guard let detailsHTML = try theme.makeTagDetailsHTML(detailsPage, context) else {
-                return
+                return nil
             }
 
-            try outputHTML(
+            return try outputHTML(
                 for: detailsPage,
                 indentedBy: indentation,
                 using: { _, _ in detailsHTML },
                 fileMode: fileMode
             )
         }
+
+        allPaths.append(contentsOf: tagPaths)
+        return allPaths
+    }
+
+    func validate(_ group: ThrowingTaskGroup<(substep: String, paths: [Path]), Error>) async throws {
+        var pathSubsteps = [Path: [String]]()
+        for try await substepAndPaths in group {
+            for path in substepAndPaths.paths {
+                if let previousSubsteps = pathSubsteps[path] {
+                    let substeps = previousSubsteps.appending(substepAndPaths.substep)
+                    throw PublishingError(
+                        path: path,
+                        infoMessage: "Path conflict in substeps: \(substeps)"
+                    )
+                }
+                pathSubsteps[path, default: []].append(substepAndPaths.substep)
+            }
+        }
     }
 
     func outputHTML<T: Location>(
@@ -133,11 +162,12 @@ private extension HTMLGenerator {
         indentedBy indentation: Indentation.Kind?,
         using generator: (T, PublishingContext<Site>) throws -> HTML,
         fileMode: HTMLFileMode
-    ) throws {
+    ) throws -> Path {
         let html = try generator(location, context)
         let path = filePath(for: location, fileMode: fileMode)
         let file = try context.createOutputFile(at: path)
         try file.write(html.render(indentedBy: indentation))
+        return path
     }
 
     func filePath(for location: Location, fileMode: HTMLFileMode) -> Path {
diff --git a/Tests/PublishTests/Tests/HTMLGenerationTests.swift b/Tests/PublishTests/Tests/HTMLGenerationTests.swift
index 2afca997..6065ea09 100644
--- a/Tests/PublishTests/Tests/HTMLGenerationTests.swift
+++ b/Tests/PublishTests/Tests/HTMLGenerationTests.swift
@@ -282,6 +282,31 @@ final class HTMLGenerationTests: PublishTestCase {
         )
     }
 
+    func testGeneratingConflictingFilesThrowsError() throws {
+        let folder = try Folder.createTemporary()
+
+        var thrownError: PublishingError?
+        do {
+            try publishWebsite(
+                in: folder,
+                using: [
+                    .addMarkdownFiles(),
+                    .generateHTML(withTheme: .foundation)
+                ],
+                content: [
+                    // This file has the same name as the `WebsiteStub.SectionID.one` case, which
+                    // causes multiple outputs at the same location.
+                    "one.md": "# One content",
+                ]
+            )
+        } catch {
+            thrownError = error as? PublishingError
+        }
+
+        let path = try require(thrownError?.path)
+        XCTAssertEqual(path, "one/index.html")
+    }
+
     func testFoundationTheme() throws {
         let folder = try Folder.createTemporary()