title: Why do geom_rect() add many multiple layers when used with facet_wrap()?
body: <p>I am adding a color bar using geom_rect() in combination with facet_wrap(), but for some reason 30 layers are added, which means the bar is completely dark even though I use alpha = 0.2.
  I can export to powerpoint and delete all the extra layes manually, but that is a huuuge pain. Is there a way to fix this problem? </p>
  
  <p>I have tried to restart the terminal and to only load the package needed, thinking maybe the error occured do to a loaded function, but no, it does not seem to be the case.</p>
  
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  0.36, 0.36, 0.36, 0.36, 0.36, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 
  0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 
  0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 
  0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.29, 0.35, 0.35, 0.35, 0.35, 
  0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 
  0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 
  0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.06, 0.06, 
  0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 
  0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 
  0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 0.06, 
  5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 
  5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 
  5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 5.21, 
  5.21, 5.21, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 
  0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 
  0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 
  0.16, 0.16, 0.16, 0.16, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 
  3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 
  3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 
  3.19, 3.19, 3.19, 3.19, 3.19, 3.19, 0.11, 0.11, 0.11, 0.11, 0.11, 
  0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 
  0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 
  0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.11, 0.16, 0.16, 0.16, 
  0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 
  0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 
  0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16, 0.16)), class = "data.frame", row.names = c(NA, 
  -770L))
  </code></pre>
  
  <p>and this is the plot:</p>
  
  <pre><code>p &lt;- ggplot(data=dat, aes(y = diff_A))+ 
    geom_boxplot(outlier.shape = 1)+
    geom_rect(aes(ymin = -MD_Fuss, ymax = MD_Fuss), xmin = -Inf, xmax =Inf, alpha = 0.2)+
    theme_bw()+ theme(panel.grid = element_blank())+ 
    xlab('')+   ylab('[mmol/L]') +
    scale_y_continuous(expand = c(0.5, 0))+
    facet_wrap(.~variable, scales = 'free')
  p
  </code></pre>