QDoc: Report duplicate warnings only once

When a project produces multiple output formats, the same broken link
can generate a warning for each format. These duplicates clutter the
warning log and can make a project exceed its warning limit even
though it has only one broken link.

This makes QDoc report an identical warning once per project run.
Distinct warnings remain visible, and the warning count reflects the
diagnostics users actually see.

Fixes: QTBUG-151404
Pick-to: 6.12
Change-Id: I358b3213e6905073b174a0abe2e1c4649c82ce1f
Reviewed-by: Topi Reinio <topi.reinio@qt.io>