If you have more than a few rows of data then this is almost impossible to do with a formula in Excel just using Excels native functions (because you need to concatenate an array, for which there is no inbuilt function).
However you can do it if you download the MOREFUNC addin and use the MCONCAT function, for your problem, with a specific SKU in D1
=SUBSTITUTE(MCONCAT(IF(A$1:A$100=D1,","&B$1:B$100,"")),",","",1)
confirmed with CTRL+SHIFT+ENTER
Download Morefunc from here
http://xcell05.free.fr/
Alternatively you could look for a VBA solution - try Googling
Harlan Grove ACONCAT
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