Remove duplicate blocks from changes_list before import

[pasqu4le]

Motivation

Solves error arising (mostly started by block_uncle fetcher) when the same block is present twice in the same changes_list

Checklist for your PR

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so if necessary
• If I added/changed/removed ENV var, I should update the list of env vars in https://github.com/poanetwork/blockscout/blob/master/docs/env-variables.md to reflect changes in the table here https://poanetwork.github.io/blockscout/#/env-variables?id=blockscout-env-variables. I've set master in the Version column.
• If I add new indices into DB, I checked, that they don't redundant with PGHero or other tools

[ayrat555]

are you sure this is only happening with uncles?

[coveralls]

Pull Request Test Coverage Report for Build e9e300a7-1bec-4456-b257-49e8d1b7bfa7

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 77.755%

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Totals
Change from base Build ed1f6586-be28-4ce7-aad1-e283ea55d6a6:	0.2%
Covered Lines:	5341
Relevant Lines:	6869

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💛 - Coveralls

[pasqu4le]

@ayrat555 it looks like it, uncles is the only one where this is happening (from the logs I have). The fix modifies the runner anyway so if this is happening with other block-inserting fetchers as well if will be de-duplicated anyway.

I am looking into why this fetcher is the only one to be affected, as like the other should have the filtering before fetching and should not end up with duplicates during importing

[ayrat555]

Enum.uniq_by?

[pasqu4le]

That would work for the duplicates, but they also need to be sorted