_coupon_in_cart_error_

What exactly triggers the _coupon_in_cart_error_ ? 

I've changed some of the language on my site text so I'm having a hard time tracing exactly what influences what as the error message may imply something different than the original. Mine currently says: "That bonus, coupon, or credit is already in your shopping cart", however it pops up when I hit the "Redeem" button on my header menu. So it's acting as if I've already input a second code that matches the first, but it doesn't allow me to input a second code it just shows the error. 

I have searched the forum and manual for this error but am getting it when I go to redeem a bonus and a bonus is already added. Can there only be one bonus/coupon applied per order?

When you try to redeem a coupon and that coupon is already in your cart.