Out of the box when you create a purchase agreement for a vendor , you can have same item multiple times in line even you can have same date range and different prices . Ideally system should not allow two different prices in same date range as in such case when PO is created always first line price is taken which is not also not correct as how can you determine which one is correct.
It should be like Trade agreements where for same vendor , same item, same date range you can not have same prices.