⌂ Repeat commands grace echo purger

One subtype of the grace synchronization issue is the grace echo. It occurs if one voice sends an (system wide collected) event (e.g. start of repeat, start or end of volta) before a grace note, while another voice does it after this grace note (usually because there is no grace note in the other voice). Then simply ignoring the second occurrence after the grace notes is a quick help.
Off course, it cannot help if there is no event before the first grace note at all, e.g. if you use a parallel control voice without grace notes to define the repeat structure.
With the \repeat and \alternative commands this method is a little tricky, because it must be done within an exact sequence of the other engravers. This example illustrates how it's done 'directly' by context modifications and using a scheme engraver, and this source code also describes how it will be implemented by patching your LILYPOND installation.