Change Page Metadata on Document Update/Insert

I'm trying to auto update the properties/metadata fields from other properties in the form tab on insert/update.

I based my solution on the example given here and came up with the following:

[assembly: RegisterModule(typeof(ApplySEOMetaDataModule))]

public class ApplySEOMetaDataModule : Module
{
    public ApplySEOMetaDataModule()
        : base("ApplySEOMetaData")
    {
    }

    protected override void OnInit()
    {
        base.OnInit();
        DocumentEvents.Update.Before += UpdateSEOFields;
        DocumentEvents.Insert.Before += UpdateSEOFields;
    }

    private static void UpdateSEOFields(object sender, DocumentEventArgs e)
    {
        if (e.Node == null)
        {
            return;
        }

        var pageDescription = e.Node.GetStringValue("DocumentPageDescription", String.Empty);
        var rollupText = GetPlainText(e.Node.GetStringValue("ListableDocumentRollupText", String.Empty));
        Logger.Write("page description: " + pageDescription);
        Logger.Write("rollup text: " + rollupText);
        if(string.IsNullOrEmpty(pageDescription)){
            e.Node.SetValue("DocumentPageDescription", rollupText);
            pageDescription = e.Node.GetStringValue("DocumentPageDescription", String.Empty);
            Logger.Write("page description after update: " + pageDescription);
        }        



        //e.Node.UpdateField("DocumentPageKeywords", (node, keywords) => keywords ?? String.Empty);
    }

    private static string GetPlainText(string text)
    {
        if (String.IsNullOrWhiteSpace(text))
        {
            return text;
        }
        return Regex.Replace(text, @"<(.|\n)*?>", string.Empty);
    }
}

However, when I check my logs, the event seems to be called multiple times:

[10/30/2017 1:00:40 PM] page description: Rollup text
[10/30/2017 1:00:40 PM] rollup text: Rollup text
[10/30/2017 1:00:40 PM] page description: Rollup text
[10/30/2017 1:00:40 PM] rollup text: Rollup text
[10/30/2017 1:00:40 PM] page description: Rollup text
[10/30/2017 1:00:40 PM] rollup text: Rollup text
[10/30/2017 1:00:40 PM] page description: Rollup text
[10/30/2017 1:00:40 PM] rollup text: Rollup text
[10/30/2017 1:00:40 PM] page description: Rollup text
[10/30/2017 1:00:40 PM] rollup text: Rollup text
[10/30/2017 1:00:40 PM] page description: 
[10/30/2017 1:00:40 PM] rollup text: Rollup text
[10/30/2017 1:00:40 PM] page description after update: Rollup text

Could someone point me in the right direction please? I know here they say to write directly to the treenode properties, but these ones only have a getter.

Thanks in advance