Generate a **distinct** list of 'categories' assigned to a set of nodes. This code loops through a set of nodes, creates an array from the categories on each node, then we get only the distinct ones. This array can then be looped through again to build a list of your categories.

Distinct list of categories.cshtml

@using Contensis.Framework.Web.Search;

@{    
    var query = Query.Where("Property_Path").StartsWith("/connect/blog/").And("Property_AuthorID").IsEqualTo(authorId.ToString()).OrderBy("Property_Title");
    var nodes = new NodeFinder().Find(query, selectCount: 20);
    string fulltagslist = "";
}

@foreach(var node in nodes)
{
    fulltagslist += node.Data.BlogCategories + ",";

}
    
@{
 string[] myfulltagslist = fulltagslist.Split(',');
 myfulltagslist = myfulltagslist.Distinct().ToArray();
}


    <ul class="sys_tagcloud-control">
       @if(myfulltagslist.Length != 1){
          foreach (string tag in myfulltagslist)
          {
              if (tag != "")
              {
      <li class="sys_tagcloud-tag"><a href="/Connect/blog/blog-archive.aspx?category=@tag">@tag</a> </li>
                  
      	}
          }
      
} else {
      			<li class="sys_tagcloud-tag">There are no categories available. </li>
      		}
      
    </ul>