[BUG][CSHARP] Duplicate assignment of nullable properties in models constructor

Bug Report Checklist

• [ ✅ ] Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• [ ✅ ] Have you tested with the latest master to confirm the issue still exists?
• [ ✅ ] Have you searched for related issues/PRs?
• [✅ ] What's the actual output vs expected output?
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Hello,

Issue is purely esthetic and doesn't affect functionality of the code.
Using version 5.4.0 of openapi generator cli it seems like it assigns model properties twice when they are covered
with "nullable" set to true.
Weird thing is that when I manually changed it to x-nullable it seem to work just fine.

Openapi-generator-cli : version 5.4.0
Openapi spec : version 3.0.1
Client lang: csharp

Example entity:

"CommentEntity": {
        "type": "object",
        "properties": {
          "commentId": {
            "type": "string",
            "format": "uuid",
            "nullable":true
          },
          "containerId": {
            "type": "string",
            "format": "uuid",
            "x-nullable": true
          }
      }

Generated model:

    /// <summary>
    /// CommentEntity
    /// </summary>
    [DataContract]
    public partial class CommentEntity :  IEquatable<CommentEntity>, IValidatableObject
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="CommentEntity" /> class.
        /// </summary>
        /// <param name="commentId">commentId.</param>
        public CommentEntity(Guid? commentId = default(Guid?),Guid? containerId = default(Guid?))
        {
            this.CommentId = commentId;
            this.CommentId = commentId;
            this.ContainerId = containerId;
        }

Expected output would look like containerId param - single assignment of property

Reproduce the issue with the supplied OpenAPI 3.0.1 schema using OpenAPI Generator CLI 5.4.0, then trace the C# model-generation path that renders the constructor. Done means each nullable property is assigned once, while the existing x-nullable behavior remains correct; check for a suitable generator regression test.