Handle duplicate refs in SubjectTypeMapping

[mchlrch]

Proposal from @nnamtug

Proposal: Handling of duplicated refs Wherever it is possible to refer the same element more than once, think about:

• not completing already referenced elements
• if applicable: validate duplicated references. this applies for 'types' and 'properties' section. example:

This makes sense for SubjectTypeMapping, as duplicate assignment makes no sense (and has no effect in the end). So, for type:

• not propose/complete already referenced elements
• validate duplicate refs with WARNING

Properties on the other hand can be referenced multiple times. For example in permit-mapping.xrm we have multiple assignments for the different languages:

properties
    schema:name from AGENCY_NAME_DE with language-tag de;
    schema:name from AGENCY_NAME_FR with language-tag fr;
    schema:name from AGENCY_NAME_EN with language-tag en;
    schema:name from AGENCY_NAME_IT with language-tag it;