No damage will result. Both power sources may exist at the same time. If there is no external power present then the P channel FET transistor allows the USB power to supply the current for the board, otherwise board power is supplied by external power, even if USB power is present.
This power source decision strategy makes sense except where if the external power source is a battery, where one would rather the board be powered from the USB, if avalible, rather then drain the battery. It does looks like if the external battey source voltage was kept between 6.7vdc (min needed before voltage regulator drop off) and 7.3vdc then the USB power, if avalible, would become the lead power source. That doesn't seem to work out to any even number of battery cells without adding some additional external diode drops.
Lefty