Merge pull request #3961 from mavasani/MultipleFiles

Ensure that we do not add duplicate public API files for multi-tfm pr…

Author: mavasani

src/PublicApiAnalyzers/Core/CodeFixes/DeclarePublicApiFix.cs

@@ -295,10 +295,15 @@ await publicSurfaceAreaAdditionalDocument.GetTextAsync(cancellationToken).Config
                     newSolution = newSolution.WithAdditionalDocumentText(pair.Key, pair.Value);
                 }
 
+                // NOTE: We need to avoid creating duplicate files for multi-tfm projects. See https://github.com/dotnet/roslyn-analyzers/issues/3952.
+                using var uniqueProjectPaths = PooledHashSet<string>.GetInstance();
                 foreach (KeyValuePair<ProjectId, SourceText> pair in addedPublicSurfaceAreaText)
                 {
                     var project = newSolution.GetProject(pair.Key);
-                    newSolution = AddPublicApiFiles(project, pair.Value);
+                    if (uniqueProjectPaths.Add(project.FilePath ?? project.Name))
+                    {
+                        newSolution = AddPublicApiFiles(project, pair.Value);
+                    }
                 }
 
                 return newSolution;