--- 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++;