diff --git a/src/addon/validation.cpp b/src/addon/validation.cpp
index 311e9dedad3b4..029000aa66d56 100644
--- a/src/addon/validation.cpp
+++ b/src/addon/validation.cpp
@@ -144,6 +144,38 @@ bool check_names_legal_internal(const config& dir, std::string current_prefix, s
 	return badlist ? badlist->empty() : true;
 }
 
+bool check_case_insensitive_duplicates_internal(const config& dir, std::string prefix, std::vector<std::string>* badlist){
+	std::set<std::string> filenames;
+	bool inserted;
+	for (const config &path : dir.child_range("file")) {
+		const config::attribute_value &filename = path["name"];
+		std::tie(std::ignore, inserted) = filenames.insert(boost::algorithm::to_lower_copy(filename.str(), std::locale::classic()));
+		if (!inserted){
+			if(badlist){
+				badlist->push_back(prefix + filename.str());
+			} else {
+				return false;
+			}
+		}
+	}
+	for (const config &path : dir.child_range("dir")) {
+		const config::attribute_value &filename = path["name"];
+		std::tie(std::ignore, inserted) = filenames.insert(boost::algorithm::to_lower_copy(filename.str(), std::locale::classic()));
+		if (!inserted) {
+			if(badlist){
+				badlist->push_back(prefix + filename.str());
+			} else {
+				return false;
+			}
+		}
+		if (!check_case_insensitive_duplicates_internal(path, prefix + filename + "/", badlist) && !badlist){
+			return false;
+		}
+	}
+
+	return badlist ? badlist->empty() : true;
+}
+
 } // end unnamed namespace 3
 
 bool check_names_legal(const config& dir, std::vector<std::string>* badlist)
@@ -155,21 +187,8 @@ bool check_names_legal(const config& dir, std::vector<std::string>* badlist)
 	return check_names_legal_internal(dir, "", badlist);
 }
 
-bool check_case_insensitive_duplicates(const config& dir){
-	std::set<std::string> filenames;
-	bool inserted;
-	for (const config &path : dir.child_range("file")) {
-		const config::attribute_value &filename = path["name"];
-		std::tie(std::ignore, inserted) = filenames.insert(boost::algorithm::to_lower_copy(filename.str(), std::locale::classic()));
-		if (!inserted) return false;
-	}
-	for (const config &path : dir.child_range("dir")) {
-		const config::attribute_value &filename = path["name"];
-		std::tie(std::ignore, inserted) = filenames.insert(boost::algorithm::to_lower_copy(filename.str(), std::locale::classic()));
-		if (!inserted) return false;
-		if (!check_case_insensitive_duplicates(path)) return false;
-	}
-	return true;
+bool check_case_insensitive_duplicates(const config& dir, std::vector<std::string>* badlist){
+    return check_case_insensitive_duplicates_internal(dir, "", badlist);
 }
 
 ADDON_TYPE get_addon_type(const std::string& str)
diff --git a/src/addon/validation.hpp b/src/addon/validation.hpp
index 4c25621ade918..009cb56e581b8 100644
--- a/src/addon/validation.hpp
+++ b/src/addon/validation.hpp
@@ -77,8 +77,21 @@ bool addon_filename_legal(const std::string& name);
  * @returns True if no illegal names were found.
  */
 bool check_names_legal(const config& dir, std::vector<std::string>* badlist = nullptr);
-/** Probes an add-on archive for case-conflicts on case-insensitive filesystems. */
-bool check_case_insensitive_duplicates(const config& dir);
+/**
+ * Scans an add-on archive for case-conflicts.
+ *
+ * Case conflicts may cause trouble on case-insensitive filesystems.
+ *
+ * @param dir     The WML container for the root [dir] node where the search
+ *                should begin.
+ * @param badlist If provided and not null, any case conflicts encountered will
+ *                be added to this list. This also makes the archive scan more
+ *                thorough instead of returning on the first conflict
+ *                encountered.
+ *
+ * @returns True if no conflicts were found.
+ */
+bool check_case_insensitive_duplicates(const config& dir, std::vector<std::string>* badlist = nullptr);
 
 std::string encode_binary(const std::string& str);
 std::string unencode_binary(const std::string& str);
diff --git a/src/campaign_server/campaign_server.cpp b/src/campaign_server/campaign_server.cpp
index ee2ef99773afc..5622ad3a5c939 100644
--- a/src/campaign_server/campaign_server.cpp
+++ b/src/campaign_server/campaign_server.cpp
@@ -693,9 +693,13 @@ void server::handle_upload(const server::request& req)
 			"File or directory names may not contain whitespace, control characters or any of the following characters: '\" * / : < > ? \\ | ~'. "
 			"It also may not contain '..' end with '.' or be longer than 255 characters.",
 			filelist, req.sock);
-	} else if(!check_case_insensitive_duplicates(data)) {
-		LOG_CS << "Upload aborted - case conflict in add-on data.\n";
-		send_error("Add-on rejected: Two files have a case conflict", req.sock);
+	} else if(!check_case_insensitive_duplicates(data, &badnames)) {
+		const std::string& filelist = utils::join(badnames, "\n");
+		LOG_CS << "Upload aborted - case conflict in add-on data (" << badnames.size() << " entries).\n";
+		send_error(
+			"Add-on rejected: The add-on contains files or directories with case conflicts. "
+			"File or directory names may not be differently-cased versions of the same string.",
+			filelist, req.sock);
 	} else if(campaign && !authenticate(*campaign, upload["passphrase"])) {
 		LOG_CS << "Upload aborted - incorrect passphrase.\n";
 		send_error("Add-on rejected: The add-on already exists, and your passphrase was incorrect.", req.sock);