Make sure we don't register the same tag twice

mpi/src/mpi/starpu_mpi_tag.c

@@ -1,7 +1,7 @@
 /* StarPU --- Runtime system for heterogeneous multicore architectures.
  *
  * Copyright (C) 2011-2017                                CNRS
- * Copyright (C) 2011-2015,2017                           Université de Bordeaux
+ * Copyright (C) 2011-2015,2017-2018                           Université de Bordeaux
  * Copyright (C) 2014                                     Inria
  *
  * StarPU is free software; you can redistribute it and/or modify
@@ -95,7 +95,12 @@ void _starpu_mpi_tag_data_register(starpu_data_handle_t handle, starpu_mpi_tag_t
 	entry->data_tag = data_tag;
 
 	_starpu_spin_lock(&registered_tag_handles_lock);
-	HASH_ADD_INT(registered_tag_handles, data_tag, entry);
+#ifndef STARPU_NO_ASSERT
+	struct handle_tag_entry *old;
+	HASH_FIND(hh, registered_tag_handles, &data_tag, sizeof(entry->data_tag), old);
+	STARPU_ASSERT_MSG(!old, "tag %"PRIi64" being registered for data %p, but is already used by data %p!\n", data_tag, handle, old->handle);
+#endif
+	HASH_ADD(hh, registered_tag_handles, data_tag, sizeof(entry->data_tag), entry);
 	_starpu_spin_unlock(&registered_tag_handles_lock);
 }