Why does the same material appear on more than one line in the purchase document?

Because the material is used in more than one work item with different units. In this case, the system splits the material into one line per unit, and each line has its quantity from the work items that use that unit.

If the unit is the same in the work items, the material quantities are combined into one line.

Example: the material Tiles is used in one work item with a quantity of 60 Pieces, in a second work item with 40 Pieces, and in a third work item with 10 Cartons. When you select the three work items, the tiles appear on two lines: one line with 100 Pieces and one line with 10 Cartons.