duplicate validation error in my custom listform template

Question

I have created a custom listform template. In this custom listform template I have removed the default <SharePoint:ListFieldIterator runat="server"/> and used my own created user control <CustomOverrideControls:CustomListFieldIterator runat="server"/>. In this user control I overwrite the CreateChildControls() method. In this method I do some logic to set some fields readonly.

public class CustomListFieldIterator : ListFieldIterator
{
    protected override void CreateChildControls()
    {
        this.Controls.Clear();
        if (this.ControlTemplate == null)
        {
            throw new ArgumentException("Could not find ListFieldIterator control template.");
        }
        for (int i = 0; i < base.Fields.Count; i++)
        {
            SPField field = base.Fields[i];
            if (!this.IsFieldExcluded(field))
            {
                var child = new CompositeField();
                child.ControlMode = base.ControlMode;
                child.FieldName = field.InternalName;
                var auditors = SPContext.Current.Web.Groups["Auditors"].ID;
                bool isAuditor = SPContext.Current.Web.IsCurrentUserMemberOfGroup(auditors);

                //if field is "Title", we in edit mode and user is not Auditor, make field readonly
                if (field.InternalName.Equals("Title") && ControlMode == SPControlMode.Edit && !isAuditor)
                {
                    child.ControlMode = SPControlMode.Display;
                }
                this.Controls.Add(new LiteralControl("<tr>"));
                this.Controls.Add(child);
                Controls.Add(new LiteralControl("</tr>"));
            }
        }

    }
}

Now I have the following problem. I have some fields which are required. If let an required field empty and press on Save, I got 2 validation warnings.

Some extra information. I am using also the ItemAdded event and do some logic. I have set the ItemAdded event to Asynchronous. I have did this because the page was not refreshing after adding a new item.

<Receivers ListTemplateId="101">
      <Receiver>
        <Name>something</Name>
        <Type>ItemAdded</Type>
        <Assembly>something</Assembly>
        <Class>something</Class>
        <SequenceNumber>10000</SequenceNumber>
        <Synchronization>Asynchronous</Synchronization>
      </Receiver>
  </Receivers>

What is the reason why I got 2 validation warnings on my form after saving with empty fields.

UPDATE: I have implemented the solution of Andrey Markeev, but it doesnt work. See below for some new information:

I have implemted your solution but it doesnt fix the duplicate validation warnings. I have undo all my changes like removing custom listform template and use the default template, change the itemadded event to asynchronous, remove my usercontrol and add the sharepoint listFieldIterator control back. I have the same problem. Do you see here something wrong:

This field has no duplicate validation error:

<Field ID="{A5A5CA1A-6CF4-4BA7-B650-7F06910EA0CD}" Name="RFCAppliesTo" StaticName="RFCAppliesTo" DisplayName="Betrekking op" Type="Text" Required="TRUE" />

These fields has duplicate validation erros. What is the problem?

<Field ID="{47705f89-4883-4ad5-8495-98ce1d4b5c35}" Name="RFCDescriptionCurrent" StaticName="RFCDescriptionCurrent" DisplayName="Omschrijving huidige situatie" Type="Note" NumLines="4" Required="TRUE" RichText="TRUE" RestrictedMode="TRUE" RichTextMode="FullHtml" IsolateStyles="TRUE" AppendOnly="FALSE" />
      <Field ID="{F61E42BC-AEC0-43AA-9AD2-AD7B36C60B22}" Name="RFCDescriptionNew" StaticName="RFCDescriptionNew" DisplayName="Omschrijving nieuwe situatie" Type="Note" NumLines="4" Required="TRUE" RichText="TRUE" RestrictedMode="TRUE" RichTextMode="FullHtml" IsolateStyles="TRUE" AppendOnly="FALSE" />
      <Field ID="{97CD7687-4F16-4AC5-9156-1F178037B5D1}" Name="RFCRisks" StaticName="RFCRisks" DisplayName="Risico's" Type="Note" NumLines="4" Required="TRUE" RichText="TRUE" RestrictedMode="TRUE" RichTextMode="FullHtml" IsolateStyles="TRUE" AppendOnly="FALSE" />
      <Field ID="{D424D1B4-4CEA-4E47-B843-6B746EAD1E35}" Name="RFCDemands" StaticName="RFCDemands" DisplayName="Eisen, randvoorwaarden, afhankelijkheden" Type="Note" NumLines="4" Required="TRUE" RichText="TRUE" RestrictedMode="TRUE" RichTextMode="FullHtml" IsolateStyles="TRUE" AppendOnly="FALSE"/>

Answer 1

The field controls aren't correctly created in your example. That's why some of the markup is most likely created twice, and that's why you end up with duplicate validator controls on the page.

Try this code:

protected override void CreateChildControls()
{
  this.Controls.Clear();
  if (this.ControlTemplate == null)
    throw new ArgumentException("Could not find ListFieldIterator control template.");

  var auditors = SPContext.Current.Web.Groups["Auditors"].ID;
  bool isAuditor = SPContext.Current.Web.IsCurrentUserMemberOfGroup(auditors);

  for (int index = 0; index < this.Fields.Count; ++index)
  {
    SPField field = this.Fields[index];
    if (!this.IsFieldExcluded(field))
    {
      TemplateContainer templateContainer = new TemplateContainer();
      this.Controls.Add((Control) templateContainer);

      //if field is "Title", we in edit mode and user is not Auditor, make field readonly
      if (field.InternalName.Equals("Title") && this.ControlMode == SPControlMode.Edit && !isAuditor)
      {
          templateContainer.ControlMode = SPControlMode.Display;
      }
      else
          templateContainer.ControlMode = this.ControlMode;

      templateContainer.FieldName = field.InternalName;
      this.ControlTemplate.InstantiateIn((Control) templateContainer);
    }
  }
}

This is based on original code from ListFieldIterator.

Answer 2

I have fixed my problem as folow:

Remove RichTextMode="FullHtml" Change RichText="TRUE" to RichText="FALSE"