One approach

In D3 enter

=IF(A3<>A2,C3,D2&"; "&C3)

Copy this down and copy the last D cell in each client number and Paste Special Values.

You could use a helper column to identify the last row in each block, e.g. =IF(A3<>A4,"Last","") and filter this column so that you can copy all the last cells in each block in one go.