She had 20 # 1 singles on the Billboard R&B Singles Chart and two # 1 hits on the Billboard Hot 100 : `` Respect '' ( 1967 ) and `` I Knew You Were Waiting ( For Me ) '' ( 1987 ) , a duet with George Michael . She has 20 # 1 singles on the Billboard R&B Singles Chart and two # 1 hits on the Billboard Hot 100 : `` Respect '' ( 1967 ) and `` I Knew You Were Waiting ( For Me ) '' ( 1987 ) , a duet with George Michael .