docs(rust): Fix some word-repetition in code comments (#14825)

Signed-off-by: geekvest <cuimoman@sohu.com>

crates/polars-core/src/frame/group_by/into_groups.rs

@@ -13,7 +13,7 @@ use crate::utils::flatten::flatten_par;
 pub trait IntoGroupsProxy {
     /// Create the tuples need for a group_by operation.
     ///     * The first value in the tuple is the first index of the group.
-    ///     * The second value in the tuple is are the indexes of the groups including the first value.
+    ///     * The second value in the tuple is the indexes of the groups including the first value.
     fn group_tuples(&self, _multithreaded: bool, _sorted: bool) -> PolarsResult<GroupsProxy> {
         unimplemented!()
     }

crates/polars-parquet/src/parquet/deserialize/filtered_rle.rs

@@ -88,7 +88,7 @@ impl<'a, I: Iterator<Item = Result<HybridEncoded<'a>, Error>>> FilteredHybridBit
     }
 
     /// Returns the number of elements remaining. Note that each run
-    /// of the iterator contains more than one element - this is is _not_ equivalent to size_hint.
+    /// of the iterator contains more than one element - this is _not_ equivalent to size_hint.
     pub fn len(&self) -> usize {
         self.total_items
     }

crates/polars/tests/it/io/csv.rs

@@ -256,7 +256,7 @@ fn test_newline_in_custom_quote_char() {
 
 #[test]
 fn test_escape_2() {
-    // this is is harder than it looks.
+    // this is harder than it looks.
     // Fields:
     // * hello
     // * ","