diff --git a/Sources/Generator/Content/Element.swift b/Sources/Generator/Content/Element.swift
index 7314dab..92b4bfe 100644
--- a/Sources/Generator/Content/Element.swift
+++ b/Sources/Generator/Content/Element.swift
@@ -319,7 +319,7 @@ struct Element {
         }
     }
 
-    func getExternalPageMap(language: String) -> [String : String] {
+    func getExternalPageMap(language: String, log: MetadataInfoLogger) -> [String : String] {
         var result = [String : String]()
         if let ext = getExternalLink(for: language) {
             result[id] = ext
@@ -327,7 +327,10 @@ struct Element {
             result[id] = path + Element.htmlPagePathAddition(for: language)
         }
         elements.forEach { element in
-            element.getExternalPageMap(language: language).forEach { key, value in
+            element.getExternalPageMap(language: language, log: log).forEach { key, value in
+                if result[key] != nil {
+                    log.error("Page id '\(key)' is used twice", source: value)
+                }
                 result[key] = value
             }
         }
diff --git a/Sources/Generator/run.swift b/Sources/Generator/run.swift
index f413496..0f4bd23 100644
--- a/Sources/Generator/run.swift
+++ b/Sources/Generator/run.swift
@@ -53,7 +53,10 @@ private func loadSiteData(in folder: URL, runFolder: URL) throws -> (root: Eleme
         print("                Error: No site root loaded, aborting generation")
         return nil
     }
-    let pageMap = root.languages.map { (language: $0.language, pages: root.getExternalPageMap(language: $0.language)) }
+    let pageMap = root.languages.map { language in
+        (language: language.language,
+         pages: root.getExternalPageMap(language: language.language, log: log))
+    }
     log.printMetadataScanOverview(languages: root.languages.count)
     return (root, pageMap)
 }