sumif with unique

Hello

I believe the function I need to use is sumif, but Can I do so by have the criteria set to unique. The req here is for column C to display a count of Column a where column B User iD is unique.

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For example in sample above provided I have 2 entries for N31529VK (Qty 37 & 34) so I need these summed to give N31529VK - 71
I have 2 entries for N42828ZZ ( Qty 29 + 29) so this should be N42828ZZ - 58

Thanks in advance

P.S I should add that I dont know every time what the User ID will be and hence using formula is not an option

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