Mixer tap issue.

Hi there, Just wondering is it normal for cold water to be pushed up the hot water pipe from the mixer tap? We have a mixer tap on our kitchen sink and when I use the hot water its fine but when I turn on the cold water it turns cold and Noticed the hot water pipe that was hot before I turned on the cold water, then the hot water pipe turned cold. Could this cause the tank in the loft/attic to over fill?


Kind regards