A unit for the incision of a strip made of a plastic material to cut two distinct incisions (7, 9) into two opposite surfaces (8, 10) of the strip in a packaging machine to manufacture a sealed single-dose break-open package; the incision unit comprises: a conveying device, which feeds the strip along a conveying direction; two support plates, which are arranged on opposite sides of the strip, so that each support plate faces a corresponding surface (8; 10) of the strip; and at least two incision devices, each of which cuts an incision (7; 9) into a corresponding surface (8; 10) of the strip and comprises a cutting element, which is supported by a support plate, and a contrast element, which is supported by the other support plate; the incision unit is characterised in that: the two support plates are mobile so as to translate together along a translation direction, which is orthogonal to the conveying direction; the two incision devices are arranged one next to the other and are aligned along the translation direction; and a first actuating device is provided, which cyclically moves the two support plates forward and backward along the translation direction between a first position, in which a first incision device (42a; 42b) is operated so as to cut a first incision (7; 9), and a second position, in which a second incision device (42b; 42a) is operated so as to cut a second incision (9; 7), which is opposite to the first incision (7, 9). a conveying device, which feeds the strip along a conveying direction; two support plates, which are arranged on opposite sides of the strip, so that each support plate faces a corresponding surface (8; 10) of the strip; and at least two incision devices, each of which cuts an incision (7; 9) into a corresponding surface (8; 10) of the strip and comprises a cutting element, which is supported by a support plate, and a contrast element, which is supported by the other support plate; the incision unit is characterised in that: the two support plates are mobile so as to translate together along a translation direction, which is orthogonal to the conveying direction; the two incision devices are arranged one next to the other and are aligned along the translation direction; and a first actuating device is provided, which cyclically moves the two support plates forward and backward along the translation direction between a first position, in which a first incision device (42a; 42b) is operated so as to cut a first incision (7; 9), and a second position, in which a second incision device (42b; 42a) is operated so as to cut a second incision (9; 7), which is opposite to the first incision (7, 9). the two support plates are mobile so as to translate together along a translation direction, which is orthogonal to the conveying direction; the two incision devices are arranged one next to the other and are aligned along the translation direction; and a first actuating device is provided, which cyclically moves the two support plates forward and backward along the translation direction between a first position, in which a first incision device (42a; 42b) is operated so as to cut a first incision (7; 9), and a second position, in which a second incision device (42b; 42a) is operated so as to cut a second incision (9; 7), which is opposite to the first incision (7, 9).