Help needed with column updates
 

I am creating a spreadsheet with rows of multiple products based on a vlookup
table. A1=Date; B1= Product Code; C1=Description; D1=Sales (Numeric); E1 to
J1=Product Pricing; K1= Opening stock (based on lookup table); L1= Remaining
stock(K1-D1).
What i would like to do is enter a formula/function to automatically enter a
value at K1 if the data at B1 is repeated or changed. I would then like the
value at L1 to be updated. I would be really grateful if anybody could
please help me with this.

billtiler