Count "x" ONLY if corresponding item number unique in other column

Hi all,

Been trying to solve this one for a while...

I have a data set with two columns: column A with all item numbers and column B with the corresponding buy "quantities" of each item, where "x" represents the quantity. Though, "x" is not actually a number, but rather stands for the general decision to buy that particular item. I now want to count the number of items that have to be bought, though, sometimes there would be accidental inputs of "x" as the same item number appears multiple times and thus shouldn't be counted more than once.

I already tried to solve it with SUMPRODUCT and COUNTIF, unfortunately the formula slows down my file by 30-40 seconds as the array is relatively big. Also, I tried to use a combination of FREQUENCY, IF, MATCH, ROW but that does not seem to be accurate as it ignores some duplicate items completely rather than counting one "x" for all duplicates of that particular item. SUMPRODUCT and COUNTIF also doesn't seem to be 100% accurate, but closer to the actual number, though, it calculates very SLOW!

I have attached a sample file where I explain the criteria further. Hopefully someone can help...

Please remember that my array is relatively large so the formula needs to be as efficient as possible.