Use a hashset to prevent attributes from being added twice.

specs/Qowaiv.CodeGeneration.Specs/Syntax/Attribute_decoration_specs.cs

@@ -20,3 +20,18 @@ public void parameter_sets()
             null, KeyValuePair.Create("Author", (object?)"Qowaiv"))
        .Should().HaveContent("[NUnit.Framework.Test(Author = \"Qowaiv\")]\r\n");
 }
+
+public class Equaly
+{
+    [Test]
+    public void by_value()
+    {
+        var attr = new AttributeInfo(typeof(TestAttribute),
+            null, KeyValuePair.Create("Author", (object?)"Qowaiv"));
+
+        var ottr = new AttributeInfo(typeof(TestAttribute),
+            null, KeyValuePair.Create("Author", (object?)"Qowaiv"));
+
+        attr.Equals(ottr).Should().BeTrue();
+    }
+}

src/Qowaiv.CodeGeneration.OpenApi/Collections/Empty.cs

@@ -4,4 +4,7 @@ internal static class Empty
 {
     [Pure]
     public static List<T> List<T>() => [];
+
+    [Pure]
+    public static HashSet<T> HashSet<T>() => [];
 }

src/Qowaiv.CodeGeneration.OpenApi/Extensions/Qowaiv.CodeGeneration.TypeInfo.cs

@@ -7,8 +7,11 @@ internal static class TypeInfoExtensions
 {
     public static void AddAttributes(this TypeInfo info, IEnumerable<AttributeInfo> attributes)
     {
-        var list = (List<AttributeInfo>)info.Attributes;
-        list.AddRange(attributes);
+        var hashset = (HashSet<AttributeInfo>)info.Attributes;
+        foreach (var attribute in attributes)
+        {
+            hashset.Add(attribute);
+        }
     }
 
     [Diagnostics.Contracts.CollectionMutation]

src/Qowaiv.CodeGeneration.OpenApi/OpenApiTypeResolver.ResolveType.cs

@@ -158,7 +158,7 @@ static bool IsNone(EnumerationField field)
                 IsPartial = Settings.Partial,
                 Properties = Empty.List<PropertyInfo>(),
                 DerivedTypes = Empty.List<Type>(),
-                Attributes = Empty.List<AttributeInfo>(),
+                Attributes = Empty.HashSet<AttributeInfo>(),
                 Visibility = ResolveVisibility(schema),
                 Documentation = new XmlDocumentation() { Summary = schema.Description },
             };

src/Qowaiv.CodeGeneration.OpenApi/Qowaiv.CodeGeneration.OpenApi.csproj

@@ -8,7 +8,7 @@
 
   <PropertyGroup>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
-    <Version>0.0.1-alpha-021</Version>
+    <Version>0.0.1-alpha-022</Version>
     <PackageId>Qowaiv.CodeGeneration.OpenApi</PackageId>
     <PackageReleaseNotes>
       <![CDATA[

src/Qowaiv.CodeGeneration/Qowaiv.CodeGeneration.csproj

@@ -8,7 +8,7 @@
 
   <PropertyGroup>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
-    <Version>0.0.1-alpha-021</Version>
+    <Version>0.0.1-alpha-022</Version>
     <PackageId>Qowaiv.CodeGeneration</PackageId>
     <PackageReleaseNotes>
       <![CDATA[

src/Qowaiv.CodeGeneration/Syntax/AttributeInfo.cs

@@ -1,3 +1,5 @@
+using DotNetProjectFile.MsBuild;
+
 namespace Qowaiv.CodeGeneration.Syntax;
 
 /// <summary>Represents an attribute decoration.</summary>
@@ -114,4 +116,10 @@ public override int GetHashCode()
 
         return hash;
     }
+
+    /// <summary>Returns true if both have the same values.</summary>
+    public static bool operator ==(AttributeInfo? l, AttributeInfo? r) => l is null ? r is null : l.Equals(r);
+
+    /// <summary>Returns false if both have the same values.</summary>
+    public static bool operator !=(AttributeInfo? l, AttributeInfo? r) => !(l == r);
 }