Using Triggers to create an audit record

I'm trying to use a trigger to generate an audit record in a new table (basically duplicate the source table). The problem that I am experiencing is trying to get a single record to be created in the target anytime the source is updated (new field entered or one updated) and I don't want 2 records to be created i.e. one for an updated field and another for an inserted field (unless these are 2 seperate operations ofcourse). The below trigger creates 2 records in hte target:

CREATE TRIGGER Tr_CAN_MARKET_DATA_AUDIT ON [dbo].[CAN_MARKET_DATA]
FOR UPDATE
AS
INSERT INTO CAN_MARKET_DATA_AUDIT
(MRKT_DATA_SEQ,
RPRTD_SRC_TXT,
CRG_VOL)
SELECT MRKT_DATA_SEQ,
RPRTD_SRC_TXT,
CRG_VOL
FROM deleted

INSERT INTO CAN_MARKET_DATA_AUDIT
(MRKT_DATA_SEQ,
RPRTD_SRC_TXT,
CRG_VOL)
SELECT MRKT_DATA_SEQ,
RPRTD_SRC_TXT,
CRG_VOL
FROM inserted

Any ideas on how to get a single instance of an updated record (inserted field or update) to generate a single audit record ?