Nd help, Vlookup/formula

Here is my current formula
=IF(ISERROR(VLOOKUP($B$99:$B$206,'Scanning
Gun'!$M$2:$O$75,2,FALSE)),0,VLOOKUP($B$99:$B$206,' Scanning
Gun'!$M$2:$O$75,2,FALSE))

Ok so what i'm trying to do is from my Scanning Gun sheet(which has part #'s
and part counts), make a link to put the part count into my main sheet. My
formula works great for that, and I made it to were my cells are blank if 0.
The problem is that if I have two of the same part numbers with different
part counts, they do not add together, instead it just takes the first part
numbers count.

I know i'm close to figuring it out, just would like to get it as fast as I
can, or atleast a step closer.
I know sumif can kinda do what i want, but my part numbers are always
changing, so no two part numbers will be the same the next day or two.

--
Thanks

Jason

Added note to my first post.

So the Scanning Gun sheet that I'm getting my information from, I believe I
need some kind of if statement there. Like if any part numbers are the same,
add their part count. I could manually do all of my cells, but that would
just be huge... I'm not for sure how to =IF(E2=E3,F2+F3,E2) this is just for
two cells, any ideas on how to make a range work in this manner.
--
Thanks

Jason