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Brother of Blues winger Eden, the Belgian was a Stamford Bridge player from 2012 until 2015.
After numerous loan spells away from the club, the midfielder joined Borussia Monchengladbach on a permanent deal.
Hazard has already scored six and assisted three times this season, equalling his goalscoring tally for the whole of last season.
And this stunning form has prompted Chelsea to reignite their interest in the player.
German newspaper Bild claim Antonio Conte is eyeing a summer move for Hazard.
Eden Hazard next club odds
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“The youngest of the Hazard family, Kylian, is also at Chelsea”
However, Gladbach are likely to be reluctant to sell.
But the draw of returning to Chelsea to play alongside his older brother may make the 24-year-old's decision an easy one.
The youngest of the Hazard family, Kylian, is also at Chelsea.
The 22-year-old has not yet featured for the Premier League champions after signing from Hungarian side Ujpest in the summer.
Thorgan, who has been capped seven times for Belgium, also failed to make a single first team appearance for Chelsea during his three years in west London.
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However, the club have form for signing players who had previously been sold.
David Luiz, Didier Drogba and Nemanja Matic all returned to Chelsea after spells away from the club.
Speaking in February, Hazard said: “The Bundesliga is a good championship and for now, I want to stay here.
"In the future maybe I will be in England again.
“I hope I can end my career in Belgium but at the moment I don’t think about the end of my career.”