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 <[email protected]>

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

Co-authored-by: Niklas Adolfsson <[email protected]>

* add migratin

* fmt

* comment typo

* some review comments

* fix bench

Co-authored-by: Niklas Adolfsson <[email protected]>
Co-authored-by: Ross Bulat <[email protected]>