Doubling vocals (or guitars) panned left/right creates an organic stereo chorus effect in and of itself. So:

• signal -> organic stereo chorus

Doubling vocals without panning them left/right creates an organic mono chorus effect. So:

• signal -> organic mono chorus

Your suggestion of mono doubled vocals with a stereo doubler effect is the same as an organic mono chorus into a synthetic stereo doubler. It's just a different effect combination.

• signal -> organic mono chorus -> synthetic stereo doubler

Organic stereo tends to be more interesting than synthetic stereo. The amount of phasing depends on many things and you're supposed to detect and fix it, if it's a problem.

Doubling vocals (or guitars) panned left/right creates an organic stereo chorus effect in and of itself. If we think that the doubling technique was an effect plugin, it would work like this:

• signal -> organic stereo chorus

Doubling vocals without panning them left/right creates an organic mono chorus effect. So:

• signal -> organic mono chorus

Your suggestion of mono doubled vocals with a stereo doubler effect is the same as an organic mono chorus into a synthetic stereo doubler. It's just a different effect combination.

• signal -> organic mono chorus -> synthetic stereo doubler

Organic stereo tends to be more interesting than synthetic stereo. The amount of phasing depends on many things and you're supposed to detect and fix it, if it's a problem.