diff --git a/archive-file.h b/archive-file.h
index 7108c7262a..496aa81ed6 100644
--- a/archive-file.h
+++ b/archive-file.h
@@ -93,6 +93,7 @@ read_thin_archive_members(C &ctx, MappedFile<C> *mf) {
     std::string path = name.starts_with('/') ?
       name : (filepath(mf->name).parent_path() / name).string();
     vec.push_back(MappedFile<C>::must_open(ctx, path));
+    vec.back()->thin_parent = mf;
     data = body;
   }
   return vec;
diff --git a/elf/main.cc b/elf/main.cc
index 27b5dc95bb..c910b5c02c 100644
--- a/elf/main.cc
+++ b/elf/main.cc
@@ -45,6 +45,7 @@ static ObjectFile<E> *new_lto_obj(Context<E> &ctx, MappedFile<Context<E>> *mf,
 
   ObjectFile<E> *file = read_lto_object(ctx, mf);
   file->priority = ctx.file_priority++;
+  file->archive_name = archive_name;
   file->is_in_lib = ctx.in_lib || (!archive_name.empty() && !ctx.whole_archive);
   file->is_alive = !file->is_in_lib;
   ctx.has_lto_object = true;
diff --git a/mold.h b/mold.h
index 7c6f9feba7..a0e8166e62 100644
--- a/mold.h
+++ b/mold.h
@@ -590,6 +590,12 @@ class MappedFile {
       // because archive members may have the same name.
       return parent->name + ":" + std::to_string(get_offset());
     }
+
+    if (thin_parent) {
+      // If this is a thin archive member, the filename part is
+      // guaranteed to be unique.
+      return thin_parent->name + ":" + name;
+    }
     return name;
   }
 
@@ -599,6 +605,7 @@ class MappedFile {
   i64 mtime = 0;
   bool given_fullpath = true;
   MappedFile *parent = nullptr;
+  MappedFile *thin_parent = nullptr;
   int fd = -1;
 };