Why is this an issue?

There’s no point in checking a variable against the value you’re about to assign it. Save the cycles and lines of code, and simply perform the assignment.

Noncompliant code example

if (x != a)  // Noncompliant; why bother?
{
    x = a;
}

Compliant solution

x = a;

Exceptions

Properties and checks inside setters are excluded from this rule because they could have side effects and removing the check could lead to undesired side effects.

if (MyProperty != a)
{
    MyProperty = a; // Compliant because the setter could be expensive call
}

private int myField;
public int SomeProperty
{
    get
    {
        return myField;
    }
    set
    {
        if (myField != value)
        {
            myField = value;
        }
    }
}