No your job cannot be replaced, but it could be a new employee may have some part of your old duties incorporated into a new job.
Say you were a chef in a cafe and were made redundant and a new waiter was taken on part of which their duties were to make the meals as well as serve them.
No of course they can't do this. Officially. But they do it via other means such as a restructuring. If you've been made unemployed and replaced with someone doing your old job you need to see a good employment solicitor. Or talk to your union.