Marinades for meat/poultry/fish should not be re-used. Particularly if it was used a couple weeks ago.
The sweetness and saltiness of teriyaki sauce would likely make it more difficult for bacterial growth but it's still a health concern and definitely not worth taking a chance on.
What you can do is use a fresh marinade as a basting glaze AFTER you have brought it to a boil for a few minutes. Then use it to glaze your meat during the last 3-5 minutes of cooking.
When you've marinated meat in mixtures that contain a lot of sweet elements you want to first wipe off the marinade, pat the meat dry and then lightly coat with a little oil before grilling/broiling, etc. Bring the marinade to a boil and then brush on as a glaze during the last few minutes of cooking. If you cook the meat with a coating of the marinade from the beginning, the sugars will caramelize and burn and you'll have the meat sticking to the grill/pan. The result will be that it tears and leaves the skin/outer layer of meat stuck to the grill when you remove it.