Which group had two UK hit singles in 1978 that each sold over two million copies?

Boney M. was a Euro-Caribbean vocal group created by German record producer Frank Farian. Originally based in West Germany, the four original members of the group's official line-up were Liz Mitchell and Marcia Barrett from Jamaica, Maizie Williams from Montserrat and Bobby Farrell, a performing artist from Aruba. The group was formed in 1976 and achieved popularity during the disco era of the late 1970s. Since the 1980s, various line-ups of the band have performed with different personnel.

In 1978, "Rivers of Babylon", a cover of a track by The Melodians with lyrics partly based on Psalm 137 and partly on Psalm 19, became (at the time) the second highest-selling single of all time in the UK. After remaining at No.1 for five weeks, "Rivers of Babylon" began dropping down the chart, at which point the B-side "Brown Girl in the Ring" was given extensive radio airplay, and the single ascended to No. 2. The single spent six months in the UK Top 40, including 19 weeks in the Top 10. It eventually sold over two million copies, the second single to do so, and is still one of only seven to achieve this feat.

The group achieved a second UK million-seller with their version of the calypso classic "Mary's Boy Child", released as a medley "Mary's Boy Child – Oh My Lord", which was previously a million-seller for Harry Belafonte. The single sold over 1.8 million copies, 1.6 million of which were in the four weeks the song was at No.1 in December 1978.