Function duplicate_domains 

pub(crate) fn duplicate_domains<T>(
    mappings: &BTreeSet<impl MappingBackendDomain<T>>,
    filter: Option<&T>,
    domain_context: Option<String>,
    domain_name: Option<&str>,
) -> Option<String>
where
    T: BackendDomainFilter,

Collects all duplicate domains.

Accepts a set of MappingBackendDomain implementations. Allows passing in an implementation of BackendDomainFilter as filter. Optionally, a domain_context and domain_name can be passed in which are used to complete the sentence “the duplicate{domain_context} {domain_name}”.