Unique Property Attribute value per class

Here is my Attribute class:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class TagAttribute : Attribute
{
    public TagAttribute (string tag)
    {
        Tag = tag;
    }

    public string Tag { get; set; }
}

The idea is that you create a class and decorate each property with the tag attribute and the tag value. For example: the property Name would have the attribute Tag("UserId")

One of the validations i need to check is that the tag value ("UserId") is unique per class properties. Meaning no other property has a tag with the same value ("UserId")

I'm pretty sure there is a simple way to do this with LINQ but casting must done too and i'd appreciate some help :)

Thanks in advance :)

This code will print all duplicate tags in a given assembly, and the list of properties that have this tag:

Assembly asm = ...
var propertiesByTag =
    from t in asm.GetTypes()
    from p in t.GetProperties()
    from a in p.GetCustomAttributes(typeof(TagAttribute)).Cast<TagAttribute>()
    group p by a.Tag into g
    select new
    {
        Tag = g.Key,
        Properties = g.ToArray()
    }

    foreach (var dup in propertiesByTag.Where(x => x.Properties.Length > 1))
    {
        Console.WriteLine("Duplicated tag: {0}", dup.Tag);
        foreach(var p in dup.Properties)
            Console.WriteLine("\t{0}.{1}", p.DeclaringType.Name, p.Name);
    }