array formula macro

Hi all,
Please bear with me on this one as its hard to explain what i need, so here goes:
I have 2 different worksheets in one workbook, lets call them sheet1 and sheet2.
Sheet 1 is data and sheet2 is results.

colum a in sheet 2 has a list of numbers approx 5000 rows long

what i need is to take sheet2 cell a1's number look for it in sheet1 colum L, when found
return the number from sheet1 colum n and place it in sheet2 cell b1, sounds easy right!!

heres the catch, the number sheet2 is looking for might be repeated multiple times in sheet1 colum L, so I would need it to add all the times it finds it together in colum n and sum the results in sheet 2 cell b1, like an array formula.

and just to chuck a spanner in the works repeat this process down the page of sheet2 untill theres nothing else to look for.

sorry for the bad explaination
any help would be great.

thanks for your reply, but would rather it was a macro

i would like to but for some reason my computer wont let me up load, illtry to make a small example here:


data steet:

colum L..............colum N
12345.................2
12345.................3
12347.................1
12347.................5
12348.................1
12348.................6

results sheet:

colum A..............colum B
12345.................(answer, search colum A, for a match in colum L (data sheet) and
..........................add the numbers from colum N (data sheet) together),so the. answer in this case in cell B1 would be 5
12347.................
12348.................

thanks