Join query query

Hi,

I'd like to do something which I think should be quite easy - that is join
2 tables and create a new table.

Table A postcode_input has columns which include postcode, eastings,
northings. there are 1,687,605 rows.
Table B bng_lat_long has columns lat, lon, e, n. There are 1,687,605 rows.

eastings = e and northings = n so there should be a 1 to 1 match. The
eastings northings pair should be unique in aggregate.

So I tried doing this:

SELECT A.postcode, A.eastings, A.northings, B.lat, B.lon INTO
postcode_lat_long
FROM postcode_input AS A
LEFT JOIN bng_lat_long AS B On A.eastings = B.e AND A.northings = B.n

And ended up with a table 13,708,233 rows long with what looks like plenty
of duplicated rows. Some but not all are duplicated. What can I do to sort
this out?

Thanks,

Andy