Breadcrumb: prevent duplicate crumbs

Since 11.2 the index sitemapitems exist (1). When the breadcrumb is built up from content beans it may find an index document in a folder. It will create two crumbs for them, but with the same link (due to the index feature). That should be prevented. Duplicate items must be filtered out.

(1) https://www.onehippo.org/library/concepts/hst-configuration-model/use-_index_-sitemap-items.html