同數據緊鄰 同編號 (14516)

B3=IF(COUNTIF(A2:A4,A3)>1,IF(A2=A3,B2,N(LOOKUP(1,0/(B$2:B2<>""),B$2:B2))+1),"")

另解=IF(A3=A2,B2,IF(A3=A4,MAX(B$2:B2)+1,""))