Like azy said...aim the flash at ceiling, but if your flash is built in sometimes having a tissue over the bulb will cut down glare.Anytime you flash a light at glass it will reflect it.Might have to ask the person to turn slightly.Or remove them. I have actually used a clone brush to remove some glare.Just take your time in postwork and work large pixel by pixel.

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."