Check for duplicate sources in messages

[zachcmadsen]

With rounded line numbers, it's possible for messages to have duplicate sources. This checks for duplicate sources before updating messages. An alternative approach is deduplicating sources after all the messages are added/updated. I went with the "on-the-fly" approach since it's simpler.

The check scans all of the sources for a message. If performance is a concern, we could try other ways to check for duplicates.

Fixes #154

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[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (fba1b0a) 90.74% compared to head (ba57994) 90.80%.

Files	Patch %	Lines
i18n-helpers/src/xgettext.rs	94.73%	0 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #156 +/- ## ========================================== + Coverage 90.74% 90.80% +0.05% ========================================== Files 11 11 Lines 2399 2436 +37 Branches 2399 2436 +37 ========================================== + Hits 2177 2212 +35 Misses 159 159 - Partials 63 65 +2 ```

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[kdarkhan]

Thanks for contribution @zachcmadsen! Feel free to pick up the followup task https://github.com/google/mdbook-i18n-helpers/issues/171 if you want.