Ensure only one instance of a particular page type can be created at a specific location

Hello,

Interesting... Maybe try use events handler ?

for example:

using CMS.Base;
using CMS.DocumentEngine;
using System;

[CustomDocumentEvents]
public partial class CMSModuleLoader
{
/// <summary>
/// Attribute class that ensures the loading of custom handlers.
/// </summary>
private class CustomDocumentEvents : CMSLoaderAttribute
{
    /// <summary>
    /// The system executes the Init method of the CMSModuleLoader attributes when the application starts.
    /// </summary>
    public override void Init()
    {
        // Assigns custom handlers to events
        DocumentEvents.Insert.Before += DocumentInsert_Before;
    }

    private void DocumentInsert_Before(object sender, DocumentEventArgs e)
    {
        var doc = e.Node;
        if (doc == null) return;
        if (doc.ClassName != "pageclassname") return;

        var quanity = DocumentHelper.GetDocuments(doc.ClassName)
            .WhereLike("NodeAliasPath", e.TargetParentNode.NodeAliasPath)
            //others conditions
            .Count;
        if (quanity > 1)
            throw new Exception("Only single instance of this page type is allowed in this location!");
    }
}
}