@java.lang.Override
  public com.google.protobuf.Duration getTombstoneTtl() {
    return tombstoneTtl_ == null
        ? com.google.protobuf.Duration.getDefaultInstance()
        : tombstoneTtl_;
  }
<pre>
The task tombstone time to live (TTL).

After a task is deleted or executed, the task's tombstone is
retained for the length of time specified by `tombstone_ttl`.
The tombstone is used by task de-duplication; another task with the same
name can't be created until the tombstone has expired. For more information
about task de-duplication, see the documentation for
[CreateTaskRequest][google.cloud.tasks.v2beta3.CreateTaskRequest.task].

Queues created by Cloud Tasks have a default `tombstone_ttl` of 1 hour.
</pre>

<code>.google.protobuf.Duration tombstone_ttl = 9;</code>

@return The tombstoneTtl.
java-tasks/proto-google-cloud-tasks-v2beta3/src/main/java/com/google/cloud/tasks/v2beta3/Queue.java