If you're trying to detect identical items, you could use the selector
/testfor @e[c=1,type=Item] {NBT}
This will only select one item (the closest, I believe). You could then use the scoreboard to tag/mark the item and do another search which excludes that item. If you are detecting the same item, but can differentiate them, such as different custom names, enchants, stacksize, etc. you can get those values via NBT. Refer to the Wiki page on NBT format for more info on the differnt tags you can use, and how to get to them.

Update for 1.13+

• The c argument has been renamed to limit.
• You can now change the sorting of the list of targets using the sort parameter. Valid values are: nearest, furthest, random, and arbitrary.
• /testfor is now /execute using the if argument.
• NBT is now inside the target selector, using the nbt argument.

1.13 command:

/execute if entity @e[limit=1,type=item,sort=nearest,nbt={NBT}]