crypto/x509: reject duplicate extensions

crypto/x509: reject duplicate extensions

When parsing certificates and CSRs, reject duplicate extensions (and
additionally duplicate requested extensions in CSRs.)

Fixes #50988

Change-Id: I531e932cfcdde78f64c106e747a68270bd4f1d80
Reviewed-on: https://go-review.googlesource.com/c/go/+/383215
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>