Not able to customize Lead screen Source dropdown

Error: The entry form (ID: CR301000, title: Leads) cannot be automated. An item with the same key has already been added.

Actual Code in CRLead DAC:

        // Summary:
        //     The source of the lead.
        //
        // Value:
        //     The field can have one of the values listed in the PX.Objects.CR.CRMSourcesAttribute
        //     class. The value of the field is automatically changed when the PX.Objects.CR.CRLead.ClassID
        //     property is changed.
        [CRMSources(BqlTable = typeof(CRLead))]
        [PXDBString(1, IsFixed = true)]
        [PXFormula(typeof(BqlOperand<Selector<classID, CRLeadClass.defaultSource>, IBqlString>.When<BqlChainableConditionBase<TypeArrayOf<IBqlBinary>.FilledWith<And<Compare<False, Equal<Use<IsImport>.AsBool>>>>>.And<Brackets<BqlChainableConditionBase<TypeArrayOf<IBqlBinary>.FilledWith<And<Compare<EntryStatus, Equal<EntryStatus.inserted>>>>>.Or<BqlOperand<Current<source>, IBqlString>.IsNull>>>>.Else<source>))]
        [PXMassMergableField]
        [PXUIField(DisplayName = "Source")]
        public override string Source { get; set; }

My DACExtension Code:

public class CRLeadExt : PXCacheExtension<PX.Objects.CR.CRLead>
    {
        #region
        [NewSources]
        [PXMergeAttributes(Method = MergeMethod.Append)]
        [PXRemoveBaseAttribute(typeof(CRMSourcesAttribute))]
        public string Source { get; set; }
        #endregion
    }