Maybe this can be a solution for you:
Enter in C2
=IF(ISERROR(MATCH(A2,A3:A7,0)),"",INDEX(B3:B7,MATCH(A2,A3:A7,0)))
and fill down as required.

> User CN Additional machine
> AA42171 Computer1 formula
> AA42171 Laptop1 |
> AB44593 Computer1 V
> AF45945 Computer1
> DF86535 Computer1
> DF86535 Computer2

Regards,
Stefi