diff --git a/dlfcn/dlerror.c b/dlfcn/dlerror.c
index 33574faab6..96bf925333 100644
--- a/dlfcn/dlerror.c
+++ b/dlfcn/dlerror.c
@@ -198,7 +198,10 @@ check_free (struct dl_action_result *rec)
       Dl_info info;
       if (_dl_addr (check_free, &info, &map, NULL) != 0 && map->l_ns == 0)
 #endif
-	free ((char *) rec->errstring);
+	{
+	  free ((char *) rec->errstring);
+	  rec->errstring = NULL;
+	}
     }
 }
 
diff --git a/malloc/Makefile b/malloc/Makefile
index 7d54bad866..e6dfbfc14c 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -38,6 +38,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
 	 tst-malloc_info \
 	 tst-malloc-too-large \
 	 tst-malloc-stats-cancellation \
+	 tst-tcfree1 tst-tcfree2 \
 
 tests-static := \
 	 tst-interpose-static-nothread \
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 67cdfd0ad2..beaab29a05 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -2888,6 +2888,8 @@ mremap_chunk (mchunkptr p, size_t new_size)
 typedef struct tcache_entry
 {
   struct tcache_entry *next;
+  /* This field exists to detect double frees.  */
+  struct tcache_perthread_struct *key;
 } tcache_entry;
 
 /* There is one of these for each thread, which contains the
@@ -2911,6 +2913,27 @@ tcache_put (mchunkptr chunk, size_t tc_idx)
 {
   tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
   assert (tc_idx < TCACHE_MAX_BINS);
+
+  /* This test succeeds on double free.  However, we don't 100% trust
+     it, so verify it's not an unlikely coincidence before
+     aborting.  */
+  if (__glibc_unlikely (e->key == tcache))
+    {
+      tcache_entry *tmp;
+      for (tmp = tcache->entries[tc_idx];
+	   tmp;
+	   tmp = tmp->next)
+	{
+	if (tmp == e)
+	  malloc_printerr ("free(): double free detected in tcache");
+	}
+      /* If we get here, it was a coincidence.  We've wasted a few
+	 cycles, but don't abort.  */
+    }
+  /* Now we mark this chunk as "in the tcache" so the above test will
+     detect a double free.  */
+  e->key = tcache;
+
   e->next = tcache->entries[tc_idx];
   tcache->entries[tc_idx] = e;
   ++(tcache->counts[tc_idx]);
@@ -2926,6 +2949,7 @@ tcache_get (size_t tc_idx)
   assert (tcache->entries[tc_idx] > 0);
   tcache->entries[tc_idx] = e->next;
   --(tcache->counts[tc_idx]);
+  e->key = NULL;
   return (void *) e;
 }
 
@@ -4173,6 +4197,21 @@ _int_free (mstate av, mchunkptr p, int have_lock)
 	tcache_put (p, tc_idx);
 	return;
       }
+    else
+      {
+	/* Check to see if it's already in the tcache.  */
+	tcache_entry *e = (tcache_entry *) chunk2mem (p);
+	/* Copy of test from tcache_put.  */
+	if (__glibc_unlikely (e->key == tcache && tcache))
+	  {
+	    tcache_entry *tmp;
+	    for (tmp = tcache->entries[tc_idx];
+		 tmp;
+		 tmp = tmp->next)
+	      if (tmp == e)
+		malloc_printerr ("free(): double free detected in tcache 2");
+	  }
+      }
   }
 #endif
 
diff --git a/malloc/tst-tcfree1.c b/malloc/tst-tcfree1.c
new file mode 100644
index 0000000000..b1258f403f
--- /dev/null
+++ b/malloc/tst-tcfree1.c
@@ -0,0 +1,40 @@
+/* Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <error.h>
+#include <limits.h>
+#include <malloc.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/signal.h>
+
+static int
+do_test (void)
+{
+  char * volatile x = malloc (32);
+
+  free (x); // puts in tcache
+  free (x); // should abort
+
+  printf("FAIL: tcache double free not detected\n");
+  return 1;
+}
+
+#define TEST_FUNCTION do_test
+#define EXPECTED_SIGNAL SIGABRT
+#include <support/test-driver.c>
diff --git a/malloc/tst-tcfree2.c b/malloc/tst-tcfree2.c
new file mode 100644
index 0000000000..2acc3036b5
--- /dev/null
+++ b/malloc/tst-tcfree2.c
@@ -0,0 +1,45 @@
+/* Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <error.h>
+#include <limits.h>
+#include <malloc.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/signal.h>
+
+static int
+do_test (void)
+{
+  char * volatile ptrs[20];
+  int i;
+
+#define COUNT 20
+  for (i=0; i<COUNT; i++)
+    ptrs[i] = malloc (20);
+  for (i=0; i<COUNT; i++)
+    free (ptrs[i]);
+  free (ptrs[0]);
+
+  printf("FAIL: tcache double free\n");
+  return 1;
+}
+
+#define TEST_FUNCTION do_test
+#define EXPECTED_SIGNAL SIGABRT
+#include <support/test-driver.c>
