Update SP - check if value exists first

Hi

I have the following SP

ALTER PROCEDURE [dbo].[sp_Trip_UpdateIgnitionOff] 
AS
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;

    -- Insert statements for procedure here
	

UPDATE tm
SET nEndEventInst = td_outer.vSequenceId, dEndTime = td_outer.dGPSDateTime
FROM tblTrip_Master tm
CROSS APPLY (
    SELECT TOP (1) td.vSequenceId, td.dGPSDateTime
    FROM tblCommonTrackingData td
    WHERE
        td.vReportId = 8 and
        --td.vSequenceId > tm.nStartEventInst AND
		td.dGPSDateTime >= tm.dStartTime and
		td.vpkDeviceID = tm.vpkDeviceID and 
		td.dGPSDateTime < DATEADD(SECOND, -30, SYSDATETIMEOFFSET())

    --ORDER BY td.vSequenceId
	ORDER BY td.dGPSDateTime, td.vSequenceId
) AS td_outer
WHERE
    tm.nEndEventInst IS NULL
	

END

Select all Open in new window

However, i've noticed an error in my data ... nEndEventInst is being put in multiple rows
It should only appear once in the table
What i need to say is something like this

WHERE
    tm.nEndEventInst IS NULL

--here is the bit i think needs changing
AND 
--do any rows with tblTrip_Master.nEndEventInst  already exist (also need to pass in [vpkDeviceID] with this)
(if exists: Select count(*) from tblTrip_Master where tblTrip_Master.vpkDeviceID  = td.vpkDeviceID AND  tblTrip_Master.nEndEventInst = tm.nEndEventInst
If COUNT  = 0 then proceed with update
Else - exit without updating

Select all Open in new window

how might I structure this?