I own a Zen Cart site selling contact lenses. For each pair of contact lenses, the customer is allow to choose two product attributes: "right eye prescription" and "left eye prescription". However, there is no difference between which one is right eye which one is left eye.
For example, a customer wants to order a pair of contact lenses in [url removed, login to view] (right eye prescription) and [url removed, login to view] (left eye prescription). Let's say I have 1000 pieces of [url removed, login to view] contact lenses instock. After the customer's purchase, I will only have 998 pieces of [url removed, login to view] contact lenses instock.
If the previous example is confusing, here is another one:
I am selling socks, but the customer can choose the size for their feet separately. There will be two attributes, one for the size of the right foot and one for the size of left foot. However, the socks for the right foot and the left foot are the same. So lets say I have 100 pieces of size 6 and size 7 socks. If a customer orders "right foot size 6 and left foot size 6", I will end up with 98 pieces of size 6 socks. Lets say another customer orders "right foot size 6 and left foot size 7", then I will end up with 97 pieces of size 6 socks, and 99 pieces of size 7 socks.
So basically, I need two identical attributes for one product, and I could keep on track of the stock for the attributes.
Please contact me if you have more questions.