html/template: check for duplicates when inserting escapers

html/template: check for duplicates when inserting escapers

Ensure that we do not insert any escapers into pipelines that
already contain an equivalent escaper. This prevents overescaping
from occuring even when an aliased parse tree that has already
been escaped is escaped again.

Fixes #21844

Change-Id: Ic00d5e01c97ef09a4e49407009cf71b0d07f5c0e
Reviewed-on: https://go-review.googlesource.com/83920
Reviewed-by: Russ Cox <rsc@golang.org>