ITAB_DUPLICATE_KEY

We are getting a "ITAB duplicate key" error in a DataSource enhancement for BW that selects data from table MBEW into a hash table.  The internal table has the same fields as the key of the table.  I don't understand how we could be getting ITAB_DUPLICATE_KEY error when the where clause on the select statement has the key of the table.  Is this a buffering issue?  Here's a subset of the important code:

    TYPES:
    BEGIN OF TY_MBEW,          "Material Valuation
      MATNR TYPE MBEW-MATNR,   "Material Number
      BWKEY TYPE MBEW-BWKEY,   "Valuation Area
      BWTAR  TYPE MBEW-BWTAR,   "Valuation Type
      STPRS TYPE MBEW-STPRS,   "Standard Price
      PEINH TYPE MBEW-PEINH,   "Price Unit
      VERPR TYPE MBEW-VERPR,   "Moving Average Price/Periodic Unit Price
      VPRSV TYPE MBEW-VPRSV,   "Price Control Indicator
    END OF TY_MBEW.

  DATA:

        IT_MBEW_HSH TYPE HASHED TABLE OF TY_MBEW
        WITH UNIQUE KEY MATNR BWKEY BWTAR,

  SELECT MATNR
         BWKEY
         BWTAR
         STPRS
         PEINH
         VERPR   "Moving Average Price/Periodic Unit Price
         VPRSV   "Price Control Indicator
    FROM MBEW
    INTO CORRESPONDING FIELDS OF TABLE it_mbew_hsh
    FOR ALL ENTRIES IN LI_DATA
    WHERE MATNR = LI_DATA-MATNR AND
          BWKEY = LI_DATA-WERKS AND
          BWTAR = ''.