feat: msig cli: Check for existing signers in add-propose

[kaola526]

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Before you mark the PR ready for review, please make sure that:

• All commits have a clear commit message.
• The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• This PR has tests for new functionality or change in behaviour
• If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• CI is green

[magik6k]

Thanks for the PR.

Code looks clean, however there's one small issue.

[magik6k]

Looks good with 1 nitpick.

Thanks for the PR!

[codecov]

Codecov Report

Merging #8833 (e3003dc) into master (d6e00bc) will decrease coverage by 0.04%.
The diff coverage is 36.84%.

❗ Current head e3003dc differs from pull request most recent head e278380. Consider uploading reports for the commit e278380 to get more accurate results

@@            Coverage Diff             @@
##           master    #8833      +/-   ##
==========================================
- Coverage   41.47%   41.43%   -0.05%     
==========================================
  Files         703      703              
  Lines       82576    82595      +19     
==========================================
- Hits        34251    34224      -27     
- Misses      42521    42558      +37     
- Partials     5804     5813       +9     

Impacted Files	Coverage Δ
cli/multisig.go	13.19% <36.84%> (+0.48%)	⬆️
storage/sealer/worker_tracked.go	80.83% <0.00%> (-6.67%)	⬇️
chain/consensus/filcns/weight.go	70.58% <0.00%> (-5.89%)	⬇️
blockstore/autobatch.go	52.94% <0.00%> (-3.37%)	⬇️
chain/sub/incoming.go	59.02% <0.00%> (-2.45%)	⬇️
blockstore/buffered.go	40.00% <0.00%> (-2.23%)	⬇️
chain/actors/builtin/power/power.go	82.60% <0.00%> (-2.18%)	⬇️
chain/sync.go	70.18% <0.00%> (-1.88%)	⬇️
markets/storageadapter/client.go	40.55% <0.00%> (-1.85%)	⬇️
chain/store/store.go	62.83% <0.00%> (-1.34%)	⬇️
... and 19 more