[DwC export]: Validate that each DwC term appears only once per mapping

[grantfitzsimmons]

Goal

Enforce that each DwC term (IRI) is mapped at most once per schema mapping. Display a warning and block saving if a term is duplicated.

Background

DwC columns in an exported file must be unique; duplicate term mappings produce invalid output and confuse downstream consumers.

Acceptance Criteria

• On Save Mapping, the system checks for duplicate term IRIs across all rows.
• If duplicates exist, a warning message lists the duplicated terms and blocks saving.
• The duplicated rows are visually highlighted in the interface.
• Users must resolve duplicates before saving.