annotation_custom failure with non-unique grob names

[baptiste]

An old issue reporting this was closed, but the behaviour is still problematic.

p <- qplot(1:10,1:10, geom="blank") + scale_x_continuous(breaks=1:10)

g <- rectGrob(gp=gpar(fill="grey"))
p + annotation_custom(g, xmin=1, xmax=2) + 
    annotation_custom(g, xmin=3, xmax=4) + # won't show up
    annotation_custom(editGrob(g, name="new"), xmin=5, xmax=6) 

I suspect it's a gtable bug, but not sure where exactly