Low pass fir filter with non-unity gain
As pointed out by niaren, you have to multiply by two, but this also means that your whole magnitude response is shifted up by 6dB, also in the stopbands. So you will have 6dB less stopband attenuation. Of course, the attenuation relative to the passband gain remains constant. You have to take this into account when designing your filter. Say you need 60dB stopband attenuation in absolute terms, not relative to the passband gain, then you need to design a 0dB gain filter with 66dB stopband gain. After multiplying the coefficients by 2 you'll get 6dB passband gain and 60dB stopband attenuation.