issue with string_agg with distinct in postgres

array_to_string(array_agg(distinct column_name::text), '; ')

Will get the job done

select string_agg(distinct 'pre' || user.col, 'post')

As the above will deny the use of an index in the distinct aggregation take the 'pre' out

select 'pre' || string_agg(distinct user.col, 'postpre')

The concat function can help you.

select string_agg(distinct concat('pre',user.col, 'post'), '')