How do you avoid comb filtering effects where there are singers or instruments with microphones spaced closely together?

Comb filter - A distortion produced by combining an acoustical (or electrical) signal with a delayed replica of itself (offset in time). The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response.

In acoustics, comb filtering can arise in some unwanted ways. For instance, when two loudspeakers are playing the same signal at different distances from the listener, there is a comb filtering effect on the signal. In any enclosed space, listeners hear a mixture of direct sound and reflected sound.

The frequency response caused by combining a sound with its delayed replica. The frequency response has a series of peaks and dips caused by phase interference. The peaks and dips resemble the teeth of a comb.

I just purchased a (used) "Studio Technologies AN-2" comb filter to simulate stereo from a mono source.
seen here:
http://www.studio-tech.com/PDFFiles/AN2ug_3.pdf

OK, this may sound stupid, but I was wondering if anyone could help me with this problem and lend some advice. It's another phase problem.