Return distinct rows based on matching data in a column in another table in PosgreSQL
Each row of e_v2 will be returned at most once by your query.  Any duplicate values you are getting are duplicates that already exist in e_v2.  You can remove them by adding a "DISTINCT" before your select-list.

SELECT DISTINCT * FROM e_v2
WHERE EXISTS (SELECT 1 FROM etraining_v1 WHERE "POINT_KEY" = etraining_v1.key)
ORDER BY 
"POINT_KEY",
"INTERVAL_READING"


Or you an clean up e_v2 so that it doesn't contain duplicates.