public Builder mergeTombstoneTtl(com.google.protobuf.Duration value) {
      if (tombstoneTtlBuilder_ == null) {
        if (((bitField0_ & 0x00000200) != 0)
            && tombstoneTtl_ != null
            && tombstoneTtl_ != com.google.protobuf.Duration.getDefaultInstance()) {
          getTombstoneTtlBuilder().mergeFrom(value);
        } else {
          tombstoneTtl_ = value;
        }
      } else {
        tombstoneTtlBuilder_.mergeFrom(value);
      }
      if (tombstoneTtl_ != null) {
        bitField0_ |= 0x00000200;
        onChanged();
      }
      return this;
    }
<pre>
The task tombstone time to live (TTL).

After a task is deleted or completed, 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.v2beta2.CreateTaskRequest.task].

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

<code>.google.protobuf.Duration tombstone_ttl = 10;</code>
java-tasks/proto-google-cloud-tasks-v2beta2/src/main/java/com/google/cloud/tasks/v2beta2/Queue.java