Deduplicate repeated sources

[mgeisler]

With #145 merged, we can now end up in a situation where a message has the same source repeated multiple times:

#: src/SUMMARY.md:10 src/SUMMARY.md:10 src/SUMMARY.md:70 src/SUMMARY.md:90
msgid "Welcome"
msgstr "Te damos la bienvenida"

We should add logic to remove the duplicates in this case. The situation becomes even worse when the line numbers are removed completely (when granularity is set to 0).

[zachcmadsen]

Hey @mgeisler, I'm interested in taking this. Based on #145, it looks like we'd need to handle duplicates whenever we call add_message in create_catalog. Am I on the right track?

[mgeisler]

Hi @zachcmadsen, thanks for taking this on! I think you're definitely on the right track!

[mgeisler]

I just created #171, which is related to this (but can be fixed independently).