Layers must have unique names

[yutannihilation]

cf. tidyverse/ggplot2#5967

> ggplot(mpg, aes(displ, hwy)) +
+   geom_point(name = "foo") +
+   gghighlight()
Error in `new_layer_names()`:
! Names must be unique.
✖ These names are duplicated:
  * "foo" at locations 1 and 2.