--- lua-5.3.4/src/lapi.c.orig	2019-06-03 17:29:46.376205064 +0200
+++ lua-5.3.4/src/lapi.c	2019-06-03 17:31:08.869905663 +0200
@@ -1289,6 +1289,8 @@
   LClosure *f1;
   UpVal **up1 = getupvalref(L, fidx1, n1, &f1);
   UpVal **up2 = getupvalref(L, fidx2, n2, NULL);
+  if (*up1 == *up2)
+    return;
   luaC_upvdeccount(L, *up1);
   *up1 = *up2;
   (*up1)->refcount++;