Answer
Try appending an OData $apply=groupby((FieldName)) query option to the LOV's underlying datasource endpoint URL, substituting the actual field name you want to group/de-duplicate by inside the parentheses. This is a standard OData aggregation option, and it's been confirmed to work for fetching distinct values from an entity set this way (for example, fetching distinct Objstate values from a customer order set with $select=Objstate&$apply=groupby((Objstate))) — while every LOV data source configuration may not have been individually verified, the pattern is a solid OData mechanism worth trying when you need distinct/grouped values rather than the raw unfiltered result set.