>> The lookup() function only works if the list you are looking up in is 
>> sorted in ascending order.  By sorting the cells by column B, 
>> concatenate(a1:a9,b1:b9) is no longer is ascending order.
>
> The question is...why?  What use is something that looks things up that 
> requires you do its work for it first?

It's because lookup always returns a "closest match" result, even if it 
can't match exactly.

Use hlookup or vlookup instead, it lets you set a flag which will only 
return exact matches, then the list does not need to be ordered.

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