Identify dublicate values

[Sunriser]

I'm facing the following problem:

In a sales table I have loaded there are several data records which appear twice. Generally, this is correct because if a customer returns a product, the purchase and the return appear as separate records (the purchase  with 'P' and return with 'R' in a separate column) but they have the same customer ID.
However, I want to eliminate both records because they don't represent real purchases.

How can I do it in Qlik Sense? Can I mark double records in a new column?

Thanks for help!

BR Sunriser

[Vu_Nguyen]

Hi,

How does your system relate a Return transaction to its original Purchase transaction? For example if you have Customer A with the following actions

CustomerID, Date, Item, Type

A, 15/11/2018, Book, P

A, 17/11/2018, Book, P

A, 20/11/2018, Book, R

How do you know that the return on 20/11 is related to the purchase on 17/11 or 15/11?

[Sunriser]

Sorry. I forgot to mention that the identifier is the product's unique serial number. So the serial number is the value which appears twice,  in your example:

A, 15/11/2018, Serial 111555, P

A, 17/11/2018, Serial 963258, P

A, 20/11/2018, Serial 111555, R

The first and third entry should be removed from my output or somehow tagged so that I can exclude it by using a filter pane. How is this possible?