diff --git a/include/cppast/cpp_entity_index.hpp b/include/cppast/cpp_entity_index.hpp
index 7e45df3..d5dd545 100644
--- a/include/cppast/cpp_entity_index.hpp
+++ b/include/cppast/cpp_entity_index.hpp
@@ -17,6 +17,7 @@
 namespace cppast
 {
     class cpp_entity;
+    class cpp_file;
     class cpp_namespace;
 
     /// \exclude
@@ -78,10 +79,17 @@ namespace cppast
         void register_definition(cpp_entity_id                           id,
                                  type_safe::object_ref<const cpp_entity> entity) const;
 
+        /// \effects Registers a new [cppast::cpp_file]().
+        /// \returns `true` if the file was not registered before.
+        /// If it returns `false`, the file was registered before and nothing was changed.
+        /// \requires The entity must live as long as the index lives.
+        /// \notes This operation is thread safe.
+        bool register_file(cpp_entity_id id, type_safe::object_ref<const cpp_file> file) const;
+
         /// \effects Registers a new [cppast::cpp_entity]() which is a declaration.
         /// Only the first declaration will be registered.
         /// \requires The entity must live as long as the index lives.
-        /// \requires The entity must not be a namespace.
+        /// \requires The entity must be forward declarable.
         /// \notes This operation is thread safe.
         void register_forward_declaration(cpp_entity_id                           id,
                                           type_safe::object_ref<const cpp_entity> entity) const;
diff --git a/include/cppast/cpp_file.hpp b/include/cppast/cpp_file.hpp
index 0ef4a11..b45ebec 100644
--- a/include/cppast/cpp_file.hpp
+++ b/include/cppast/cpp_file.hpp
@@ -50,11 +50,11 @@ namespace cppast
 
             /// \effects Registers the file in the [cppast::cpp_entity_index]().
             /// It will use the file name as identifier.
-            /// \returns The finished file.
+            /// \returns The finished file, or `nullptr`, if that file was already registered.
             std::unique_ptr<cpp_file> finish(const cpp_entity_index& idx) noexcept
             {
-                idx.register_definition(cpp_entity_id(file_->name()), type_safe::ref(*file_));
-                return std::move(file_);
+                auto res = idx.register_file(cpp_entity_id(file_->name()), type_safe::ref(*file_));
+                return res ? std::move(file_) : nullptr;
             }
 
         private:
diff --git a/include/cppast/parser.hpp b/include/cppast/parser.hpp
index 708cc99..63ac895 100644
--- a/include/cppast/parser.hpp
+++ b/include/cppast/parser.hpp
@@ -84,6 +84,7 @@ namespace cppast
 
         /// \effects Parses the given file.
         /// \returns The [cppast::cpp_file]() object describing it.
+        /// It can be `nullptr`, if there was an error or the specified file already registered in the index.
         /// \requires The dynamic type of `config` must match the required config type.
         std::unique_ptr<cpp_file> parse(const cpp_entity_index& idx, std::string path,
                                         const compile_config& config) const
diff --git a/src/cpp_entity_index.cpp b/src/cpp_entity_index.cpp
index da46bb2..521e367 100644
--- a/src/cpp_entity_index.cpp
+++ b/src/cpp_entity_index.cpp
@@ -7,6 +7,7 @@
 #include <cppast/detail/assert.hpp>
 #include <cppast/cpp_entity.hpp>
 #include <cppast/cpp_entity_kind.hpp>
+#include <cppast/cpp_file.hpp>
 
 using namespace cppast;
 
@@ -33,6 +34,13 @@ void cpp_entity_index::register_definition(cpp_entity_id
     }
 }
 
+bool cpp_entity_index::register_file(cpp_entity_id                         id,
+                                     type_safe::object_ref<const cpp_file> file) const
+{
+    std::lock_guard<std::mutex> lock(mutex_);
+    return map_.emplace(std::move(id), value(file, true)).second;
+}
+
 void cpp_entity_index::register_forward_declaration(
     cpp_entity_id id, type_safe::object_ref<const cpp_entity> entity) const
 {
diff --git a/src/libclang/libclang_parser.cpp b/src/libclang/libclang_parser.cpp
index 44bcb85..cafede5 100644
--- a/src/libclang/libclang_parser.cpp
+++ b/src/libclang/libclang_parser.cpp
@@ -399,7 +399,7 @@ std::unique_ptr<cpp_file> libclang_parser::do_parse(const cpp_entity_index& idx,
     auto              macro_iter   = preprocessed.macros.begin();
     auto              include_iter = preprocessed.includes.begin();
 
-    // convert entity hierachies
+    // convert entity hierarchies
     detail::parse_context context{tu.get(), file, type_safe::ref(logger()), type_safe::ref(idx),
                                   detail::comment_context(preprocessed.comments)};
     detail::visit_tu(tu, path.c_str(), [&](const CXCursor& cur) {
@@ -444,5 +444,5 @@ std::unique_ptr<cpp_file> libclang_parser::do_parse(const cpp_entity_index& idx,
 catch (detail::parse_error& ex)
 {
     logger().log("libclang parser", ex.get_diagnostic());
-    return cpp_file::builder(path).finish(idx);
+    return nullptr;
 }