--- tester.cc   92f084120ecb09cf0c92c3f291070cfe250a88b8
+++ tester.cc   c6bbcfe068040e5d4eef9b7ffc66a8de05d49fd4
@@ -413,11 +413,13 @@ LUAEXT(set_env, )
 {
   char const * var = luaL_checkstring(L, -2);
   char const * val = luaL_checkstring(L, -1);
-  char const * old = getenv(var);
-  if (old)
-    orig_env_vars.insert(make_pair(string(var), string(old)));
-  else
-    orig_env_vars.insert(make_pair(string(var), ""));
+  if (orig_env_vars.find(string(var)) == orig_env_vars.end()) {
+    char const * old = getenv(var);
+    if (old)
+      orig_env_vars.insert(make_pair(string(var), string(old)));
+    else
+      orig_env_vars.insert(make_pair(string(var), ""));
+  }
   setenv(var, val);
   return 0;
 }