diff --git a/lldb/source/Symbol/ClangASTImporter.cpp b/lldb/source/Symbol/ClangASTImporter.cpp
--- a/lldb/source/Symbol/ClangASTImporter.cpp
+++ b/lldb/source/Symbol/ClangASTImporter.cpp
@@ -882,6 +882,14 @@
 
 void ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(
     clang::Decl *to, clang::Decl *from) {
+  // We might have a forward declaration from a shared library that we
+  // gave external lexical storage so that Clang asks us about the full
+  // definition when it needs it. In this case the ASTImporter isn't aware
+  // that the forward decl from the shared library is the actual import
+  // target but would create a second declaration that would then be defined.
+  // We want that 'to' is actually complete after this function so let's
+  // tell the ASTImporter that 'to' was imported from 'from'.
+  MapImported(from, to);
   ASTImporter::Imported(from, to);
 
   /*
diff --git a/lldb/unittests/Symbol/TestClangASTImporter.cpp b/lldb/unittests/Symbol/TestClangASTImporter.cpp
--- a/lldb/unittests/Symbol/TestClangASTImporter.cpp
+++ b/lldb/unittests/Symbol/TestClangASTImporter.cpp
@@ -88,6 +88,33 @@
   EXPECT_EQ(origin.decl, source.record_decl);
 }
 
+TEST_F(TestClangASTImporter, CompleteFwdDeclWithOtherOrigin) {
+  // Create an AST with a full type that is defined.
+  clang_utils::SourceASTWithRecord source_with_definition;
+
+  // Create an AST with a type thst is only a forward declaration with the
+  // same name as the one in the the other source.
+  std::unique_ptr<TypeSystemClang> fwd_decl_source = clang_utils::createAST();
+  CompilerType fwd_decl_type = clang_utils::createRecord(
+      *fwd_decl_source, source_with_definition.record_decl->getName());
+
+  // Create a target and import the forward decl.
+  std::unique_ptr<TypeSystemClang> target = clang_utils::createAST();
+  ClangASTImporter importer;
+  CompilerType imported = importer.CopyType(*target, fwd_decl_type);
+  ASSERT_TRUE(imported.IsValid());
+  EXPECT_FALSE(imported.IsDefined());
+
+  // Now complete the forward decl with the definition from the other source.
+  // This should define the decl and give it the fields of the other origin.
+  clang::TagDecl *imported_tag_decl = ClangUtil::GetAsTagDecl(imported);
+  importer.CompleteTagDeclWithOrigin(imported_tag_decl,
+                                     source_with_definition.record_decl);
+  ASSERT_TRUE(imported.IsValid());
+  EXPECT_TRUE(imported.IsDefined());
+  EXPECT_EQ(1U, imported.GetNumFields());
+}
+
 TEST_F(TestClangASTImporter, DeportDeclTagDecl) {
   // Tests that the ClangASTImporter::DeportDecl completely copies TagDecls.
   clang_utils::SourceASTWithRecord source;