In ATG once used coupon not getting disassociated after the completion of the order. If without ending the session I am continuing same discount is reflected in cart without using the coupon.
Coupon is just a delivery mechanism of a promotion. All that a coupon functionality does is to allow user to enter it and identify the promotion associated with the coupon. It depends on the promotion flags and rules to determine whether this promotion is only for this order or can it be used multiple times or stays on forever.
In your case, you may have a promotion with "Max Uses per Customer" more than 1. This will allow for the customer to get this promotion in his profile and every time he/she comes back it will be available for use.
Check this out for information - http://docs.oracle.com/cd/E24152_01/Platform.10-1/ATGMerchandisingUserGuide/html/s1404availabilitycouponstab01.html
If you feel that the promotion is defined correctly, look at the activePromotions property on the profile to find why it continues to remain active. Here is more information on that - http://docs.oracle.com/cd/E24152_01/Platform.10-1/ATGCommStoreGuide/html/s0502howpromotionswork01.html