How to restrict total allocation percentage of a resource to be 100%

We have a requirement to restrict the allocation percentage of resources. When we allocate resources to a project, we specify the allocation percentage. Now, we want to restrict it there.

For instance, if a resource is allocated 50% to a project, then during that time period, it should only be allocated 50% to any other project. The total allocation percentage across all projects should remain 100% for that resource.

Currently, what happens is that a resource is fully allocated(100 %) to one project, and even after that, we can allocate them 100% to other projects. So, we need to impose this restriction.