All idrefs in an attribute must be unique.
Affects: @who
@ed-who
@roles
@src
@lexicon
@morphology
@reuse-type
@bitext-relation
@feature
@include
@licensor
@period
@rights
@work
@type
@adverb
@group
@verb
@where
@relationship
@topic
@subject
@object
@units
Used by template ŧ core-expansion-terse-attributes
.