Question: Who most commonly sang the fastest-selling single by an Australian artist as a duet?
Context: John Vickery (1906-1983) was an Australian artist.  He worked in the fields of painting, illustration and advertising.  He is the only Australian artist to have been a part of the New York School. Roy De Maistre CBE (27 March 18941 March 1968) was an Australian artist of international fame.  He is renowned in Australian art for his early experimentation with "colour-music", and is recognised as the first Australian artist to use pure abstraction.  His later works were painted in a figurative style generally influenced by Cubism.  His "Stations of the Cross" series hangs in Westminster Cathedral and works of his are hung in the Tate Gallery, London and in the Art Gallery of New South Wales, Sydney. Anthony Cosmo Callea (born 13 December 1982) is an Australian singer-songwriter and stage actor of Italian descent (parents Santina and Cosmo) who rose to prominence after being the runner-up in the 2004 season of "Australian Idol".  Callea's debut single "The Prayer" is the fastest-selling single by an Australian artist, held the No.1 spot on the ARIA Singles Chart for five weeks, a record for the debut single of an "Australian Idol" contestant, and was the second-highest selling Australian single of the last decade. "The Prayer" is a popular song written by David Foster, Carole Bayer Sager, Alberto Testa and Tony Renis.  It is most commonly known as a duet between Celine Dion and Andrea Bocelli.  It is from "Quest for Camelot: Devojka od čokolade is the second studio album by Serbian recording artist Ana Nikolić.  It was released in 2006 by City Records, and most of its songs were written by Aleksandar Milić Mili, Ljiljana Jorgovanović and Marina Tucaković, who worked on Nikolić's debut release Januar from 2003.  The album became one of the fastest-selling in the region and its most notable single was "Romale Romali", that Ana sang on Beovizija in 2006 and was written by Kiki Lesendrić.  Video for the main title song of the album won the audience sympathies immediately and so did the ballad “Plakaćete za mnom oboje” as well as songs “Dum jedan konjak”, “Klovn”, “Verna do kolena”. Dannielle Gaha DeAndrea, sometimes styled as Danni'elle, is an Australian singer-songwriter.  She has toured and recorded with fellow Australian John Farnham; their 1989 duet, "Communication", peaked at number 13 on the Australian Recording Industry Association (ARIA) Singles Chart.  When based in London from 1992 to 1993, she released three solo singles, "Stuck in the Middle", "Do It for Love", and "Secret Love" which appeared in the UK Singles Chart, and she is the first Australian artist to appear on the UK soul chart. Gulgardi is a 1971 painting by Kaapa Mbitjana Tjampitjinpa, an Indigenous Australian artist from Papunya in Australia's Northern Territory.  It is notable for being the first work by an Indigenous Australian artist to win a contemporary art award, and the first public recognition of a Papunya painting. Maria Malibran (24 March 180823 September 1836) was a Spanish mezzo-soprano who commonly sang both contralto and soprano parts, and was one of the most famous opera singers of the 19th century.  Malibran was known for her stormy personality and dramatic intensity, becoming a legendary figure after her death at age 28.  Contemporary accounts of her voice describe its range, power and flexibility as extraordinary. Shayne Thomas Ward (born 16 October 1984) is an English singer and actor of Irish Traveller Heritage, who rose to fame as the winner of the second series of "The X Factor".  His debut single, "That's My Goal", was released in the United Kingdom on 21 December 2005 and reached number one on the UK Singles Chart and was that year's Christmas number one.  It sold 313,000 copies on its first day of sales, making it the third fastest-selling single of all time in the UK, behind Elton John's "Candle in the Wind 1997" and Will Young's "Evergreen", which sold 685,000 and 400,000 copies in their first days of sale respectively. Twenty Ten is a two-disc retrospective album by Australian singer and songwriter Guy Sebastian.  Disc one contains 18 songs from his previous five albums and two new songs, including the lead single "Who's That Girl" which features American rapper Eve.  Disc two has acoustic versions of ten of the songs.  The album was released in Australia on 19 November 2010 and reached number four on the ARIA Album Chart, Sebastian's sixth consecutive top ten album.  It was certified platinum in its fourth week of release and was the fifth highest selling Australian artist album of 2010.  It reached 2× platinum certification in 2014.  "Who's That Girl" reached number one on the ARIA Singles Chart and achieved 5× platinum certification, and was Sebastian's fifth of six number one singles in Australia.  It won the Highest Selling Single ARIA Award in 2011, and received nominations for Single of the Year and Best Pop Release.  Sebastian was also nominated as Most Popular Australian Artist.  "Who's That Girl" also reached number one and Platinum certification in New Zealand. <ref name="Latest Gold/Platinum Singles"> .  RadioScope New Zealand.  Archived from the original on 24 July 2011.  Retrieved 15 January 2012. </ref>
Answer: Celine Dion and Andrea Bocelli