Duplicate InternalName of the Site column

A duplicate name "<string>" was found. at Microsoft.SharePoint.SPFieldCollection.AddFieldToWeb(String strXml, Boolean checkDisplayName) at
Microsoft.SharePoint.SPFieldCollection.AddFieldAsXmlInternal(String schemaXml, Boolean addToDefaultView, SPAddFieldOptions op) at Microsoft.SharePoint.Deployment.FieldTemplateSerializer.CreateField(SPWeb web, SerializationInfoHelper infoHelper) at Microsoft.SharePoint.Deployment.FieldTemplateSerializer.SetObjectData(Object obj, SerializationInfo info, StreamingContext context, ISurrogateSelector selector) at
Microsoft.SharePoint.Deployment.XmlFormatter.ParseObject(Type objectType, Boolean isChildObject) at Microsoft.SharePoint.Deployment.XmlFormatter.DeserializeObject(Type objectType, Boolean isChildObject, DeploymentObject envelope) at
Microsoft.SharePoint.Deployment.XmlFormatter.Deserialize(Stream serializationStream) at Microsoft.SharePoint.Deployment.ObjectSerializer.Deserialize(Stream
serializationStream) at Microsoft.SharePoint.Deployment.ImportObjectManager.ProcessObject(XmlReader xmlReader) at Microsoft.SharePoint.Deployment.SPImport.DeserializeObjects() at Microsoft.SharePoint.Deployment.SPImport.Run()" Time="2008-07-22T22:37:58.4082752Z"
Severity="Error" Phase="ImportInProgress"

To resolve this issue, you need to deactivate & uninstall the Feature the feature and modify the Fields.xml. You need to change the one of the Site Column name. Activate & Install  the feature again.

Steps to Deactivate & Uninstall the Feature
=======================================

C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN>stsadm -o DeactivateFeature -name {FeatureName} -url {SiteCollection’s Url}

C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN>stsadm -o uninstallFeature -name {FeatureName}

Steps to Install and Activate the Feature
=====================================
C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN>stsadm -o installFeature -name {FeatureName}

C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN>stsadm -o ActivateFeature -name {FeatureName} -url {SiteCollection’s Url}

Note :- If your site is highly customized and it could be possible that you are referring these column in the object model. If anywhere the code is referring to “InternalName” Property of the these Site Column, then these changes could break your Business logic. I would strongly recommend that before doing these change, please check the source code of your customized “.dll” installed on the site .