The two suspects bailed after three women were alleged to have been held as slaves for 30 years were previously arrested in the 1970s.
Police would not say why the man and woman, both 67, were arrested, or whether they were convicted at the time - adding that the current investigation "will take considerable time".
The pair have been arrested on suspicion of immigration offences as well as in connection with the investigation into slavery and domestic servitude, Scotland Yard said.
The victims - a 30-year-old British woman, a 57-year-old Irish woman and a 69-year-old Malaysian woman - are in the care of a specialist non-governmental organisation after they were rescued from a house in Lambeth, south London, last month.
It is thought the 30-year-old woman had been in servitude all her life.
Police said that over many decades the suspects and the victims would "probably have come into contact with public services", including the Metropolitan Police.
The case came to light after the "very distressed" Irish woman rang a charity to say she had been held against her will in a house in London for more than 30 years.
Police would not reveal the nationality of the two suspects but said they had been in the country for "many years".
They said the case "so far is unique to us" and it was a "complicated and disturbing picture of emotional control over many years", in which the women were restrained by "invisible handcuffs".
Detective Inspector Kevin Hyland, from the human trafficking unit said: "We know that there has been physical abuse, described as beatings."
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