What happens when a computer is plugged into another computer by USB?
It should not damage your laptop as long as the power bank keeps working normally (gives approximately 5V).
Of course the voltage from the laptop and the power bank can never be exactly same, but these interfaces are safe for use with slight voltage differences (up to 5.5 V should be definitely safe). There is usually no path for any significant amount of current to flow backwards into a USB port of a computer, except when the voltage rises to unusually high values, e.g. 12V.