public interface IFunctionality1
{
  void DoStuff(IReadOnlyList<Person> persons, IReadOnlyList<Case> cases);
}

public interface IFunctionality2
{
    void DoMoreStuff(IReadOnlyList<Person> persons, IReadOnlyList<Case> cases);
}

public class ActualFunctionality1 : IFunctionality1
{
  public void DoStuff(IReadOnlyList<Person> persons, IReadOnlyList<Case> cases)
  {
    cases = MergeRelatedCases(cases);

    //code block for the first functionality should have been moved here
  }

  private IReadOnlyList<Case> MergeRelatedCases(IReadOnlyList casesToMerge)
  {
    //code to do the merging goes here
  }
}

public class ActualFunctionality2 : IFunctionality2
{
  public void DoStuff(IReadOnlyList<Person> persons, IReadOnlyList<Case> cases)
  {
    //code block for the second functionality should have been moved here
  }
}

//common variables used accross all functionalities
var functionality1 = ObjectFactory.GetInstance<IFunctionality1>();
var functionality2 = ObjectFactory.GetInstance<IFunctionality2>();

var persons = personsRepo.GetPersons(filterParameters);
var cases = casesRepo.GetCases();


//call the updated component for the first functionality here
//now it does not change the shared variable anymore
functionality1(persons, cases)

//call the component for the second functionality here
functionality2(persons, cases)