refactor: Prevent duplicate schema serialization errors in AsyncAPI components

The serialization logic in AsyncApiComponents.cs is updated to remove the explicit distinct filtering when writing schemas for JSON and MultiFormat types. This change addresses a regression where inlining recursive schemas reached by multiple references previously caused an error ("An item with the same key has already been added").
This change is supported by updating the serialization logic in src/ByteBard.AsyncAPI/Models/AsyncApiComponents.cs to remove the distinct filtering logic for AsyncApiJsonSchemaReference and AsyncApiMultiFormatSchemaReference when writing schemas.
A corresponding regression test in test/ByteBard.AsyncAPI.Tests/Models/AsyncApiSchema_Should.cs has been added to verify that serializing components with recursive references reached multiple times no longer throws an error.

Author: VisualBean

src/ByteBard.AsyncAPI/Models/AsyncApiComponents.cs

@@ -2,7 +2,6 @@ namespace ByteBard.AsyncAPI.Models
 {
     using System;
     using System.Collections.Generic;
-    using System.Linq;
     using ByteBard.AsyncAPI.Models.Interfaces;
     using ByteBard.AsyncAPI.Writers;
 
@@ -129,9 +128,6 @@ public void SerializeV2(IAsyncApiWriter writer)
                 writer.WriteStartObject();
                 if (loops.TryGetValue(typeof(AsyncApiJsonSchemaReference), out List<object> schemas))
                 {
-                    var asyncApiSchemas = schemas.Cast<AsyncApiJsonSchemaReference>().Distinct().ToList()
-                        .ToDictionary<AsyncApiJsonSchemaReference, string>(k => k.Reference.FragmentId);
-
                     writer.WriteOptionalMap(
                        AsyncApiConstants.Schemas,
                        this.Schemas,
@@ -395,9 +391,6 @@ public void SerializeV3(IAsyncApiWriter writer)
                 writer.WriteStartObject();
                 if (loops.TryGetValue(typeof(AsyncApiMultiFormatSchemaReference), out List<object> schemas))
                 {
-                    var asyncApiSchemas = schemas.Cast<AsyncApiMultiFormatSchemaReference>().Distinct().ToList()
-                        .ToDictionary<AsyncApiMultiFormatSchemaReference, string>(k => k.Reference.FragmentId);
-
                     writer.WriteOptionalMap(
                        AsyncApiConstants.Schemas,
                        this.Schemas,

test/ByteBard.AsyncAPI.Tests/Models/AsyncApiSchema_Should.cs

@@ -712,5 +712,47 @@ public void V2_Serialize_WithNot_DoesNotWriteElse()
 
             Assert.True(!yaml.Contains("else:"));
         }
+
+        /// <summary>
+        /// Regression test.
+        /// Bug: Inlining a recursive schema that is reached by more than one reference threw
+        /// "An item with the same key has already been added" when writing components.
+        /// </summary>
+        [Test]
+        public void V2_Serialize_WithInlinedReferences_RecursiveSchemaReachedTwice_DoesNotThrow()
+        {
+            var yaml =
+                """
+                asyncapi: 2.6.0
+                info:
+                  title: test
+                  version: 1.0.0
+                channels:
+                  first:
+                    publish:
+                      message:
+                        payload:
+                          $ref: '#/components/schemas/Node'
+                components:
+                  schemas:
+                    Node:
+                      type: object
+                      properties:
+                        next:
+                          $ref: '#/components/schemas/Node'
+                        prev:
+                          $ref: '#/components/schemas/Node'
+                """;
+            var document = new AsyncApiStringReader().Read(yaml, out var diagnostic);
+            diagnostic.Errors.Should().BeEmpty();
+
+            var outputString = new StringWriter();
+            var writer = new AsyncApiYamlWriter(outputString, new AsyncApiWriterSettings { ReferenceInline = ReferenceInlineSetting.InlineReferences });
+
+            var act = () => document.SerializeV2(writer);
+
+            act.Should().NotThrow();
+            outputString.ToString().Replace("\r\n", "\n").Should().Contain("components:\n  schemas:\n    Node:");
+        }
     }
 }