How can I stop additional geoms chaning the key alpha in ggplot2?

I have two ggplot2::geom_pologons filled with a partially transparent colour.
I would like the key to display with the same alpha, but it does not.
The solution would ideally scale such that as I add additional polygons with the same fill and alpha colour the matching holds.

Example code:

chosen_alpha = 0.2
ggplot() +
    theme_minimal() +
    geom_polygon(
        aes(
            fill = "A",
            x = c(1, 2, 2),
            y = c(1, 1, 2)
        ),
        alpha = chosen_alpha
    ) +
    geom_polygon(
        aes(
            fill = "A",
            x = c(3, 4, 4),
            y = c(3, 3, 4)
        ),
        alpha = chosen_alpha
    ) +
    guides(
        fill = guide_legend(
            override.aes = list(
                alpha = chosen_alpha
            ),
        )
    )

This does work if I only have one of the polygons, as follows.

chosen_alpha = 0.2
ggplot() +
    theme_minimal() +
    geom_polygon(
        aes(
            fill = "A",
            x = c(1, 2, 2),
            y = c(1, 1, 2)
        ),
        alpha = chosen_alpha
    ) +
    guides(
        fill = guide_legend(
            override.aes = list(
                alpha = chosen_alpha
            ),
        )
    )

The issue is that for each geom a key glyph is drawn, i.e. you end up with multiple layers of slightly transparent rects which are drawn on top of each other and hence you end up with a darker color the more geoms are added.

The cleanest approach to fix your issue would be to use only one geom_polygon to draw your polygons. To this end put your data in a data.frame and account for the different polygons via the group aes.

library(ggplot2)

chosen_alpha <- 0.2

dat <- data.frame(
  x = c(c(1, 2, 2), c(2, 3, 3)),
  y = c(c(1, 1, 2), c(2, 2, 3)),
  fill = "A",
  group = rep(1:2, each = 3)
)
ggplot(dat) +
  theme_minimal() +
  geom_polygon(
    aes(
      fill = fill,
      x = x,
      y = y,
      group = group
    ),
    alpha = chosen_alpha
  )

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Second option would be to set the key_glyph to "blank" for all but one geom. This way the key glyph is only drawn once:

ggplot() +
  theme_minimal() +
  geom_polygon(
    aes(
      fill = "A",
      x = c(1, 2, 2),
      y = c(1, 1, 2)
    ),
    alpha = chosen_alpha,
    key_glyph = "blank"
  ) +
  geom_polygon(
    aes(
      fill = "A",
      x = c(2, 3, 3),
      y = c(2, 2, 3)
    ),
    alpha = chosen_alpha
  )

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