remove duplicate accounts in chain-spec. (#7725)

* remove duplicate accounts in chain-spec.

* Fix build

* Enforce in balances modules.

* Apply suggestions from code review

* Update frame/balances/src/lib.rs

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

substrate/frame/balances/src/lib.rs

@@ -412,6 +412,12 @@ decl_storage! {
 					"the balance of any account should always be at least the existential deposit.",
 				)
 			}
+
+			// ensure no duplicates exist.
+			let endowed_accounts = config.balances.iter().map(|(x, _)| x).cloned().collect::<std::collections::BTreeSet<_>>();
+
+			assert!(endowed_accounts.len() == config.balances.len(), "duplicate balances in genesis.");
+
 			for &(ref who, free) in config.balances.iter() {
 				T::AccountStore::insert(who, AccountData { free, .. Default::default() });
 			}

substrate/frame/balances/src/tests.rs

@@ -639,6 +639,15 @@ macro_rules! decl_tests {
 			}.assimilate_storage(&mut t).unwrap();
 		}
 
+		#[test]
+		#[should_panic = "duplicate balances in genesis."]
+		fn cannot_set_genesis_value_twice() {
+			let mut t = frame_system::GenesisConfig::default().build_storage::<$test>().unwrap();
+			let _ = GenesisConfig::<$test> {
+				balances: vec![(1, 10), (2, 20), (1, 15)],
+			}.assimilate_storage(&mut t).unwrap();
+		}
+
 		#[test]
 		fn dust_moves_between_free_and_reserved() {
 			<$ext_builder>::default()