How to prevent duplicate files to be uploaded in to Document libray in sharepoint

scenario :
I want to  prevent to add duplicate files to document library in sharepoint . when we are uploading document it will verify the all Doclib Files then if file name is exists it will show alertbox.

Then I have written some sample code for this issue:

/// <summary>

       /// An item is being added.

       /// </summary>

       public override void ItemAdding(SPItemEventProperties properties)

       {

           string fileUrl = properties.BeforeUrl;

           string fileName = fileUrl.Substring(fileUrl.LastIndexOf('/') + 1);

           if (IsExisted(properties.List, fileName))

           {

                 properties.Cancel = true;

               properties.Status = SPEventReceiverStatus.CancelWithRedirectUrl;

               properties.RedirectUrl = "/_Layouts/MyEventError.aspx";

           }

       }

       /// <summary>

       /// An item is being updated.

       /// </summary>

       public override void ItemUpdating(SPItemEventProperties properties)

       {

           string fileUrl = properties.BeforeUrl;

           string fileName = fileUrl.Substring(fileUrl.LastIndexOf('/') + 1);

           if (IsExisted(properties.List, fileName))

           {

               properties.Cancel = true;

               properties.Status = SPEventReceiverStatus.CancelWithRedirectUrl;

               properties.RedirectUrl = "/_Layouts/MyEventError.aspx";

           }

       }

       private Boolean IsExisted(SPList list, string fileName)

       {

           bool isExisted = false;

           SPQuery query = new SPQuery();

           query.Query = string.Format(

               "<Where><Eq><FieldRef Name='FileLeafRef'></FieldRef><Value Type='Text'>{0}</Value></Eq></Where>",

               fileName);

           query.ViewAttributes = "Scope=\"Recursive\"";

           SPListItemCollection itemColl = list.GetItems(query);

           if (itemColl.Count > 0)

           {

               isExisted = true;

           }

           return isExisted;

       }