From 0d09ef6df75158b54aaf0a84525953502cd79e50 Mon Sep 17 00:00:00 2001
From: Matthias Koefferlein <matthias@klayout.de>
Date: Sun, 19 Oct 2025 18:34:54 +0200
Subject: [PATCH] Refinement: do not merge library cells that are already
 present

---
 src/lay/lay/layLibraryController.cc | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/lay/lay/layLibraryController.cc b/src/lay/lay/layLibraryController.cc
index 7324ead45..bbdcc8869 100644
--- a/src/lay/lay/layLibraryController.cc
+++ b/src/lay/lay/layLibraryController.cc
@@ -99,10 +99,14 @@ private:
     std::vector<db::cell_index_type> target_cells, source_cells;
 
     //  collect the cells to pull in (all top cells of the library layout)
+    //  NOTE: cells are not overwritten - the first layout wins, in terms
+    //  of cell names and also in terms of database unit.
     for (auto c = ly.begin_top_down (); c != ly.end_top_cells (); ++c) {
       std::string cn = ly.cell_name (*c);
-      source_cells.push_back (*c);
-      target_cells.push_back (layout ().add_cell (cn.c_str ()));
+      if (! layout ().has_cell (cn.c_str ())) {
+        source_cells.push_back (*c);
+        target_cells.push_back (layout ().add_cell (cn.c_str ()));
+      }
     }
 
     db::CellMapping cm;