A situation in a polyphonic synth where two (or more) voices are inadvertently playing the same note. Double voicing is usually undesirable because it can cause phase cancellations or other intermodulation effects, which will make the double-voiced note sound out of place in a chord. Normally double voicing is not possible when playing from a keyboard; it usually results from either a MIDI transmission error, or from a sequencer improperly overlapping MIDI note on and note off messages. Double voicing often leads to a stuck note situation when the note is released, requiring sending an all notes off message, or power cycling the effected synth, to get rid of it.
Double voicing may be desirable in the context of a guitar synthesizer. A characteristic of a guitar is that chords can be voiced such that two or more strings are playing the same note, and generally the performer will want the synth to follow the guitar's voicing.