How do I select distinct json columns?
You'll want PostGIS's ST_GeomFromGeoJSON().

SELECT DISTINCT ST_GeomFromGeoJSON(geojson)
FROM tbl;


Should work to get the points. However may find this is faster... I'd be interested in seeing a benchmark. ST_Union will eliminate duplicate points and geometries too.

SELECT ST_Union(ST_GeomFromGeoJSON(geojson))
FROM tbl;


Update

Is it faster, what advantages and disadvantages does this have?


DISTINCT requires a SORT. faster to sort floats than strings.
My ST_Mem_Size(ST_GeomFromGeoJSON(geojson)) shows 32 bytes (two floats).
Not sure how much space the raw JSON takes, likely more than 32 bytes.
Likely have to have these as ::geography at some time.
Processing the JSON and creating points is a one time cost but it may be significant.
ST_GeomFromJSON supports all geometry objects. Including MultiPoints.


Update2

If you already got the type as startstationgeom, just do

SELECT ST_AsGeoJSON(ST_Union(startstationgeom)) FROM table


That will output only unique geometries.