In BenchmarkDup fuction, heap is created as h := make(myHeap, n)
and then n elements are added, so first time there are 2*n elements
in heap.

Fixes #15380

Change-Id: I0508486a847006b3cd545fd695e8b09af339134f
Reviewed-on: https://go-review.googlesource.com/22310Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>