Looking up a brake like bulb for a 2.5 impreza RS on rockauto.com says 1157, which is indeed a dual filament bulb.
WAGNER Part # BP1157 {Miniature Lamp-Blister Pack of 2}
The 1157 is a dual filament bulb while the 1156 is a single filament bulb. They look very similar but are not interchangeable. A common mistake is to put an 1156 into a socket for an 1157 bulb. This causes all kinds of weird problems like dash lights going on when you step on the brake. You can see in the pictures below that the 1157 has two brass terminals in the base while the 1156 has only one. The 1157 bulb is also keyed so it will only go into the socket one way. Note that one lock tab on the 1157 is higher than the other. This prevents it from going in wrong. Also note that the lock tabs on the 1156 are the same height so it will not go into an 1157 socket.
:shrug: