How to build validation to check occurrence of a member in a rollup

Summary:

Need to have a validation to check that a member has been added in a Alternate Rollup only once.

Content (please ensure you mask any confidential information):

We have multiple Alternate Rollups in a viewpoint, want to ensure that a member is not added to same Alternate Rollup twice.

Version (include the version you are using, if applicable):

24.12

Code Snippet (add any code snippets that support your topic, if applicable):