Total dodge width depends on unique order values

[teunbrand]

This PR aims to fix #6775.

Briefly, when order has duplicates, these were considered as 'separate' groups to be accommodated and contributed to the width computation. But because they overlap, the width computation can ignore duplicates.
This PR just counts order if available, and else counts group.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

df <- data.frame(
  y = c(1, 3, 2),
  g = c("A", "B", "C"),
  o = c(1, 2, 2)
)

ggplot(df, aes("x", y, fill = g, order = o)) +
  geom_col(position = position_dodge(preserve = "single"))

^{Created on 2025-12-18 with reprex v2.1.1}