Prevent proposals with duplicate actions from being submitted in `GovernorTimelockCompound`

[Amxx]

It is a know issue in Compound Governance that proposal queueing will fail if it contains 2 (or more) actions that have the same target, value, and calldata. While this can be circumvented by adding extra (useless) bytes at the end of the calldata to create unicity, it is possible to create such a proposal and not realised if will never be executable after the voting ends.

We should modify the behavior of _propose in GovernorTimelockCompound to reject proposals that have duplicated actions

[gabrielrondon]

Hi, I'd like to work on this. Here's my proposed approach:

Override _propose in GovernorTimelockCompound to detect duplicate actions at proposal creation time, before the voting period begins. Pairwise O(n²) comparison of (target, value, keccak256(calldata)) tuples — negligible cost since action arrays are typically single digits.

New error GovernorDuplicateProposalAction(uint256 index) declared in the extension contract, following GUIDELINES.md convention. Reports the index of the first duplicate found.

Override _propose (not propose) since NatSpec says it's the intended extension point — also covers proposeBySig.

I have a branch ready with tests passing. Happy to adjust based on feedback.