Fourier filtering of periodic image creates double frequency?

I want to clean a very periodic image (interferogram) from noise by Fourier filtering. Therefore I compute the Fourier transform, mask the carrier frequency of the fringes symmetrically around the center, and compute the inverse Fourier transform. The filtered image appears to have a periodicity of the doubled frequency. The code looks generally like this:

fourier = fft2(image);
fourier = fftshift(image);
fourierMasked = fourier.*mask;
fourierMasked = ifftshift(fourierMasked);
imageFiltered = ifft2(fourierMasked);

This effect does not occur if the masked area (which is the area I use for the ifft) does contain the pixels of minimum frequency (with using fftshift the central pixels). But I really don't want to take the low frequency noise into the filtered image.

Any idea, why the doubling of the frequency appears and how to circumvent it?

Thanks, Christine