I wont reuse the milk once it is "warmed up" to room temperature from fridge temperature. Tho you did not warm it up using a warmer, it is warmed up when you leave it at room temperature.
Some pple will reuse the milk cos it is not exposed to bacteria as the bottle is not opened but I wont take chances.
For some pple, if baby cannot finish milk, they put the unfinished milk back to fridge and give to baby at next feed. but i dont feel comfy with it.