Properly filter out duplicate voters in elections. (#6693)

* Prevent duplicate voter

* Update primitives/npos-elections/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/npos-elections/src/lib.rs

@@ -371,6 +371,10 @@ pub fn seq_phragmen<AccountId, R>(
 	voters.extend(initial_voters.into_iter().map(|(who, voter_stake, votes)| {
 		let mut edges: Vec<Edge<AccountId>> = Vec::with_capacity(votes.len());
 		for v in votes {
+			if edges.iter().any(|e| e.who == v) {
+				// duplicate edge.
+				continue;
+			}
 			if let Some(idx) = c_idx_cache.get(&v) {
 				// This candidate is valid + already cached.
 				candidates[*idx].approval_stake = candidates[*idx].approval_stake

substrate/primitives/npos-elections/src/tests.rs

@@ -588,6 +588,66 @@ fn self_votes_should_be_kept() {
 	);
 }
 
+#[test]
+fn duplicate_target_is_ignored() {
+	let candidates = vec![1, 2, 3];
+	let voters = vec![
+		(10, 100, vec![1, 1, 2, 3]),
+		(20, 100, vec![2, 3]),
+		(30, 50, vec![1, 1, 2]),
+	];
+
+	let ElectionResult { winners, assignments } = seq_phragmen::<_, Perbill>(
+		2,
+		2,
+		candidates,
+		voters,
+	).unwrap();
+	let winners = to_without_backing(winners);
+
+	assert_eq!(winners, vec![(2), (3)]);
+	assert_eq!(
+		assignments
+			.into_iter()
+			.map(|x| (x.who, x.distribution.into_iter().map(|(w, _)| w).collect::<Vec<_>>()))
+			.collect::<Vec<_>>(),
+		vec![
+			(10, vec![2, 3]),
+			(20, vec![2, 3]),
+			(30, vec![2]),
+		],
+	);
+}
+
+#[test]
+fn duplicate_target_is_ignored_when_winner() {
+	let candidates = vec![1, 2, 3];
+	let voters = vec![
+		(10, 100, vec![1, 1, 2, 3]),
+		(20, 100, vec![1, 2]),
+	];
+
+	let ElectionResult { winners, assignments } = seq_phragmen::<_, Perbill>(
+		2,
+		2,
+		candidates,
+		voters,
+	).unwrap();
+	let winners = to_without_backing(winners);
+
+	assert_eq!(winners, vec![1, 2]);
+	assert_eq!(
+		assignments
+			.into_iter()
+			.map(|x| (x.who, x.distribution.into_iter().map(|(w, _)| w).collect::<Vec<_>>()))
+			.collect::<Vec<_>>(),
+		vec![
+			(10, vec![1, 2]),
+			(20, vec![1, 2]),
+		],
+	);
+}
+
 mod assignment_convert_normalize {
 	use super::*;
 	#[test]