egui_plots: Fix the same plot tick label being painted multiple times (#4307)

Usually this isn't visible (the same label being painted on top of
itself), but it will be visible if the user has a custom formatter (e.g.
`y_axis_formatter`) that choses a different format based on
`GridMark:step_size` (e.g. using fewer decimals for thicker ticks).

crates/egui_plot/src/lib.rs

@@ -13,7 +13,7 @@ mod memory;
 mod plot_ui;
 mod transform;
 
-use std::{ops::RangeInclusive, sync::Arc};
+use std::{cmp::Ordering, ops::RangeInclusive, sync::Arc};
 
 use egui::ahash::HashMap;
 use egui::*;
@@ -1717,9 +1717,31 @@ fn generate_marks(step_sizes: [f64; 3], bounds: (f64, f64)) -> Vec<GridMark> {
     fill_marks_between(&mut steps, step_sizes[0], bounds);
     fill_marks_between(&mut steps, step_sizes[1], bounds);
     fill_marks_between(&mut steps, step_sizes[2], bounds);
+
+    // Remove duplicates:
+    // This can happen because we have overlapping steps, e.g.:
+    // step_size[0] =   10  =>  [-10, 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120]
+    // step_size[1] =  100  =>  [     0,                                     100          ]
+    // step_size[2] = 1000  =>  [     0                                                   ]
+
+    steps.sort_by(|a, b| match cmp_f64(a.value, b.value) {
+        // Keep the largest step size when we dedup later
+        Ordering::Equal => cmp_f64(b.step_size, a.step_size),
+
+        ord => ord,
+    });
+    steps.dedup_by(|a, b| a.value == b.value);
+
     steps
 }
 
+fn cmp_f64(a: f64, b: f64) -> Ordering {
+    match a.partial_cmp(&b) {
+        Some(ord) => ord,
+        None => a.is_nan().cmp(&b.is_nan()),
+    }
+}
+
 /// Fill in all values between [min, max] which are a multiple of `step_size`
 fn fill_marks_between(out: &mut Vec<GridMark>, step_size: f64, (min, max): (f64, f64)) {
     debug_assert!(max > min);