EPM: allow duplicate submissions (#12237)

* allow for duplicate signed submissions

* Fix a bunch of things, seems all good now

* fmt

* Fix

* Update frame/election-provider-multi-phase/src/signed.rs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* Update frame/election-provider-multi-phase/src/signed.rs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* add migratin

* fmt

* comment typo

* some review comments

* fix bench

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>
Co-authored-by: Ross Bulat <ross@parity.io>

substrate/frame/election-provider-multi-phase/src/mock.rs

@@ -571,6 +571,12 @@ impl ExtBuilder {
 			balances: vec![
 				// bunch of account for submitting stuff only.
 				(99, 100),
+				(100, 100),
+				(101, 100),
+				(102, 100),
+				(103, 100),
+				(104, 100),
+				(105, 100),
 				(999, 100),
 				(9999, 100),
 			],