Fix S1939: Extends and implements list entries should not be redundant

[michalb-sonar]

The analyzer throws exception for following (invalid) code:

public class Foo : IList, IList { }

Exception:

Analyzer 'SonarAnalyzer.Rules.CSharp.RedundantInheritanceList' threw an exception of type 'System.ArgumentException' with message 'An item with the same key has already been added.'.
'Exception occurred with following context:

System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at SonarAnalyzer.Common.MultiValueDictionaryExtensions.ToMultiValueDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector)
   at SonarAnalyzer.Rules.CSharp.RedundantInheritanceList.GetImplementedInterfaceMappings(BaseListSyntax baseList, SemanticModel semanticModel)
   at SonarAnalyzer.Rules.CSharp.RedundantInheritanceList.ReportRedundantInterfaces(SyntaxNodeAnalysisContext context, BaseTypeDeclarationSyntax typeDeclaration)
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