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 var materialStd={flow:123, name:'Raoul'};
 var obj={
 id:0,
 line:[
 {id:1, material:materialStd},
 {id:2, material:materialStd},
 {id:3, material:materialStd}]
 }
 // Compare them
 console.log(
 (obj.line[0].material === obj.line[1].material)
 &&
 (obj.line[1].material === obj.line[2].material)
 ) // true

[Because pass by reference][1].

 var materialStd={flow:123, name:'Raoul'};
 var obj={
 id:0,
 line:[
 {id:1, material:materialStd},
 {id:2, material:materialStd},
 {id:3, material:materialStd}]
 }
 // Compare them
 console.log(
 (obj.line[0].material === obj.line[1].material)
 &&
 (obj.line[1].material === obj.line[2].material)
 ) // true

As suggested by other answers, clone a fresh reference for your intended behaviour inside obj.line[index]["material"] [1]: https://medium.com/nodesimplified/javascript-pass-by-value-and-pass-by-reference-in-javascript-fcf10305aa9c

Because pass by reference.

 var materialStd={flow:123, name:'Raoul'};
 var obj={
 id:0,
 line:[
 {id:1, material:materialStd},
 {id:2, material:materialStd},
 {id:3, material:materialStd}]
 }
 // Compare them
 console.log(
 (obj.line[0].material === obj.line[1].material)
 &&
 (obj.line[1].material === obj.line[2].material)
 ) // true

As suggested by other answers, clone a fresh reference for your intended behaviour inside obj.line[index]["material"]