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>