Alert Popping for more number of times

HI

We set an Query Based alert which is popping for more number of times even we set the frequency for 1 day @ 11 am.

We set the users too.

At 11,the users are receiving the same alert for 12 or 13 times.Dont know the exact reason.

here is the aleret query,

Select E.DocNum,A.DocEntry ,A.LineNum , E.DocDate, A.ItemCode , A.Dscription ,A.Quantity , A.OpenCreQty, A.U_Status  ,
CASE when E.U_PrePO = 'yes' then qc.AcceptedQty else isnull(a.U_AppQty,0) end as AcceptedQty,
CASE when E.U_PrePO = 'yes' then qc.RejectedQty  else isnull(a.U_RejQty,0) end as RejectedQty,
CASE when E.U_PrePO = 'yes' then qc.TestedQty  else isnull(a.U_UsedQty,0)end as TestedQty
from PDN1 A 
left outer join (
Select B.U_DocEntry, B.U_DocDesc,B.U_DocNum,B.U_LineNo,B.U_ItemCode,sum(isnull(C.U_AceptQty,0))as AcceptedQty, sum(isnull(C.U_RejctQty,0))as RejectedQty,(sum(isnull(C.U_RejctQty,0))+ sum(isnull(C.U_AceptQty ,0)))as TestedQty
from [@QCTRANHEAD] B inner join [@QCTRANDET] C on B.Code = C.Code
Group By B.U_DocEntry, B.U_DocDesc,B.U_DocNum,B.U_LineNo,B.U_ItemCode) QC
on A.DocEntry = QC.U_DocEntry and A.LineNum = QC.U_LineNo and A.ItemCode = QC.U_ItemCode 
inner join OPDN E on A.DocEntry = E.DocEntry 
where A.U_Status = 'closed'
and CASE when E.U_PrePO = 'yes' then qc.RejectedQty  else isnull(a.U_RejQty,0) end <> 0
and E.UpdateDate = CONVERT(date,getdate())