Im trying to create a check-out for an engraving website.

my session array:
key=product, value=quantity, value=text to be engraved

everythings cool until customers ordering same key(product) but different value as in different text to be engraved on same product line. This is permited, ordering two or more pens but with different engraving.

the problem is that session would write over the same key value. How do i go about being able to take order of same key but different value and be able to write both into session?