Handle Case When Validator is Assigned to Propose and Attest in Same Slot

[rauljordan]

Given the small parameter values of committees in our local testing environment, sometimes the situation arises when a validator is assigned as both a proposer and attester in the same slot. Discussed offline, we should handle this case and allow the validator to perform both responsibilities.

[0xKiwi]

I'd like to handle this! 😄