Oracle Event Alert firing multiple times

Hi

I have an Event Alert which needs to fired when the PO gets "REJECTED"

but to my surprise it is firing multiple times for particular PO

Alert Type : Event

Table : PO_HEADERS_ALL

AFTER UPDATE

Code:

    SELECT

      pha.attribute13,

      nvl(pha.rate,1),

      ( pla.unit_price * pla.quantity ) po_price,

      pha.org_id,

      pha.po_header_id

    INTO

      &P_AFE_NUMBER,

    &P_RATE,

    &P_PO_PRICE,

    &P_ORGANIZATION_ID,

    &P_PO_HEADER_ID

    FROM

      po_headers_all pha,

      po_lines_all  pla

    WHERE

        pha.po_header_id = pla.po_header_id

      AND pha.authorization_status = 'REJECTED'

      AND TRUNC (pha.last_update_date) = TRUNC (SYSDATE)