Why is this an issue?

Passing a collection as an argument to the collection’s own method is a code defect. Doing so might either have unexpected side effects or always have the same result.

Another case is using set-like operations. For example, using Union between a list and itself will always return the same list. Conversely, using Except between a list and itself will always return an empty list.

var list = new List<int>();

list.AddRange(list);          // Noncompliant
list.Concat(list);            // Noncompliant

list.Union(list);             // Noncompliant: always returns list
list.Intersect(list);         // Noncompliant: always returns list
list.Except(list);            // Noncompliant: always returns empty
list.SequenceEqual(list);     // Noncompliant: always returns true

var set = new HashSet<int>();
set.UnionWith(set);           // Noncompliant: no changes
set.IntersectWith(set);       // Noncompliant: no changes
set.ExceptWith(set);          // Noncompliant: always returns empty
set.SymmetricExceptWith(set); // Noncompliant: always returns empty
set.IsProperSubsetOf(set);    // Noncompliant: always returns false
set.IsProperSupersetOf(set);  // Noncompliant: always returns false
set.IsSubsetOf(set);          // Noncompliant: always returns true
set.IsSupersetOf(set);        // Noncompliant: always returns true
set.Overlaps(set);            // Noncompliant: always returns true
set.SetEquals(set);           // Noncompliant: always returns true

Resources

Documentation

• Collections