prevent repeated items

A couple ways you can do this....

1) PreFix the List with the specific category of items to be added:

Laptop , 5 , HP
Playstation, 1, Sony
PC 2, etc...
Printer 3, HP

This way you will always exepect 1 list category, but might not work if multiple options exist.. for this case you can create profiles based on other conditions and populate the fixed categores to be added within the List. only.

2) The second option will include to have the populate from a existing list where will filter only in what is available will require a little trickery and outbound rules to loop and determine if between the two lists what is remaining and only make accessible the next one in line.

3) If you only need a validation create and outbound rule that loop trough the list and make sure on the category there is no duplicated fields otherwise popup alert and stop executing rules.

Hope this helps.