Prevent duplicate item in a SharePoint List

The following Event Handler code will prevent users from creating duplicate value in  "Title" field.

ItemAdding Event Handler

 public override void ItemAdding(SPItemEventProperties properties)
        {
            base.ItemAdding(properties);
            if (properties.ListTitle.Equals("My List"))
            {
                try
                {
                    using(SPSite thisSite = new SPSite(properties.WebUrl))
                    {
                        SPWeb thisWeb = thisSite.OpenWeb();
                        SPList list = thisWeb.Lists[properties.ListId];
                        SPQuery query = new SPQuery();
                        query.Query = @"<Where><Eq><FieldRef Name='Title' /><Value Type='Text'>" + properties.AfterProperties["Title"] + "</Value></Eq></Where>";
                        SPListItemCollection listItem = list.GetItems(query);
                        if (listItem.Count > 0)
                        {
                            properties.Cancel = true;
                            properties.ErrorMessage = "Item with this Name already exists. Please create a unique Name.";
                        }
                    }
                }
                catch (Exception ex)
                {
                    PortalLog.LogString("Error occured in event ItemAdding(SPItemEventProperties properties)() @ AAA.BBB.PreventDuplicateItem class. Exception Message:" + ex.Message.ToString());
                    throw new SPException("An error occured while processing the My List Feature. Please contact your Portal Administrator");
                }
            }
        }


Feature.xml


<?xml version="1.0" encoding="utf-8"?>
<Feature  Id="1c2100ca-bad5-41f5-9707-7bf4edc08383"
          Title="Prevents Duplicate Item"
          Description="Prevents duplicate Name in the "My List" List"
          Version="12.0.0.0"
          Hidden="FALSE"
          Scope="Web"
          DefaultResourceFile="core"
          xmlns="http://schemas.microsoft.com/sharepoint/">
  <ElementManifests>
    <ElementManifest Location="elements.xml"/>
  </ElementManifests>
</Feature>

Element.xml


<?xml version="1.0" encoding="utf-8" ?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <Receivers ListTemplateId="100">
    <Receiver>
      <Name>AddingEventHandler</Name>
      <Type>ItemAdding</Type>
      <SequenceNumber>10000</SequenceNumber>
      <Assembly>AAA.BBB, Version=1.0.0.0, Culture=neutral, PublicKeyToken=8003cf0cbff32406</Assembly>
      <Class>AAA.BBB.PreventDuplicateItem</Class>
      <Data></Data>
      <Filter></Filter>
    </Receiver>
  </Receivers>
</Elements>

The source code of this feature is available at Technet Gallery