autonumberhash256 merely creates a hash value. It does not check for existing values. For that you need a where clause.
LOAD Autonumberhash256(Order_ID,DATETIME) as primarykey, ...etc...
WHERE Not Exists(primarykey,Autonumberhash256(Order_ID,DATETIME));
SELECT .... FROM ... WHERE DATETIME>sysdate;
If QVDlocation does not exist then records are loaded from an sql source. The next statement will concatenate records to that. If those records come from the same sql source then you'll likely be loading some records twice.