default Tuple2<Seq<T>, Seq<T>> duplicate() {
        return duplicate(this);
    }
Duplicate a Streams into two equivalent Streams.
<p>
<pre><code>
// tuple((1, 2, 3), (1, 2, 3))
Seq.of(1, 2, 3).duplicate()
</code></pre>

@see #duplicate(Stream)
jOOL/src/main/java/org/jooq/lambda/Seq.java