You can use the Where Condition with Exists() function to avoid loading the primary Keys which are already loaded.
Have a look at the link Understanding EXISTS() function - Qlikview vs SQL/RDBMS
Hi Naveen, there are many documents explaining techniques to use incremental load: Incremental Load in Qlikview - Sources
To give only one link (this is included in the list posted above): Incremental Load Scenarios.pdf
Your case may vary depending if the data can be updated in DB after reload, in that case you'll need to check the Last_modification_date and the keys, if this is not the case and once the data is written in db it can't be updated, you only need to check if you have loaded the same key before to avoid the load of duplicates.