Add row in Grid

Hi, I´m Tried Add a new Registry in a Grid, but thats registry replace another registry en the grid.

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private Set listaItemsVenta;

//method add new Row in the Grid
//But in the Set if permanent boot registry with de diferrent Item number. Example:
//registry 1: item → 1 id-> 1010 name → producto1 sellPrice → 300 Quantity → 2
//registry 2: item → 2 id-> 1010 name → producto1 sellPrice → 300 quantity → 3
//But in the grid show 2 registry equals
// registry 2: item → 2 id-> 1010 name → producto1 sellPrice → 300 quantity → 3
// registry 2: item → 2 id-> 1010 name → producto1 sellPrice → 300 quantity → 3

item++;
pventaDTO.setItem(item);
pventaDTO.setCantidadVenta(nfCantidad.getValue());
listaItemsVenta.add(pventaDTO);
grid.getDataProvider().refreshAll();
`