commit 7098efff946437a2db6013d12c4fc3193fc328ce
Author: Tony Cook <tony@develop-help.com>
Date:   2016-01-27 11:52:15 +1100

    remove duplicate environment variables from environ
    
    If we see duplicate environment variables while iterating over
    environ[]:
    
    a) make sure we use the same value in %ENV that getenv() returns.
    
    Previously on a duplicate, %ENV would have the last entry for the name
    from environ[], but a typical getenv() would return the first entry.
    
    Rather than assuming all getenv() implementations return the first entry
    explicitly call getenv() to ensure they agree.
    
    b) remove duplicate entries from environ
    
    Previously if there was a duplicate definition for a name in environ[]
    setting that name in %ENV could result in an unsafe value being passed
    to a child process, so ensure environ[] has no duplicates.

diff --git perl.c perl.c
index d3e378f..25e2075 100644
--- perl.c
+++ perl.c
@@ -4281,23 +4281,70 @@ S_init_postdump_symbols(pTHX_ int argc, char **argv, char **env)
 	}
 	if (env) {
 	  char *s, *old_var;
+          STRLEN nlen;
 	  SV *sv;
+          HV *dups = newHV();
+
 	  for (; *env; env++) {
 	    old_var = *env;
 
 	    if (!(s = strchr(old_var,'=')) || s == old_var)
 		continue;
+            nlen = s - old_var;
 
 #if defined(MSDOS) && !defined(DJGPP)
 	    *s = '\0';
 	    (void)strupr(old_var);
 	    *s = '=';
 #endif
-	    sv = newSVpv(s+1, 0);
-	    (void)hv_store(hv, old_var, s - old_var, sv, 0);
+            if (hv_exists(hv, old_var, nlen)) {
+                const char *name = savepvn(old_var, nlen);
+
+                /* make sure we use the same value as getenv(), otherwise code that
+                   uses getenv() (like setlocale()) might see a different value to %ENV
+                 */
+                sv = newSVpv(PerlEnv_getenv(name), 0);
+
+                /* keep a count of the dups of this name so we can de-dup environ later */
+                if (hv_exists(dups, name, nlen))
+                    ++SvIVX(*hv_fetch(dups, name, nlen, 0));
+                else
+                    (void)hv_store(dups, name, nlen, newSViv(1), 0);
+
+                Safefree(name);
+            }
+            else {
+                sv = newSVpv(s+1, 0);
+            }
+	    (void)hv_store(hv, old_var, nlen, sv, 0);
 	    if (env_is_not_environ)
 	        mg_set(sv);
 	  }
+          if (HvKEYS(dups)) {
+              /* environ has some duplicate definitions, remove them */
+              HE *entry;
+              hv_iterinit(dups);
+              while ((entry = hv_iternext_flags(dups, 0))) {
+                  STRLEN nlen;
+                  const char *name = HePV(entry, nlen);
+                  IV count = SvIV(HeVAL(entry));
+                  IV i;
+                  SV **valp = hv_fetch(hv, name, nlen, 0);
+
+                  assert(valp);
+
+                  /* try to remove any duplicate names, depending on the
+                   * implementation used in my_setenv() the iteration might
+                   * not be necessary, but let's be safe.
+                   */
+                  for (i = 0; i < count; ++i)
+                      my_setenv(name, 0);
+
+                  /* and set it back to the value we set $ENV{name} to */
+                  my_setenv(name, SvPV_nolen(*valp));
+              }
+          }
+          SvREFCNT_dec_NN(dups);
       }
 #endif /* USE_ENVIRON_ARRAY */
 #endif /* !PERL_MICRO */