Don't create duplicate translations with status waiting+warning during imports

[ocean90]

I noticed this on translate.w.org. We sync plugin translations from a dev to a stable project. Example flow:

• https://translate.wordpress.org/projects/wp-plugins/nothing-much/dev/de/default?filters[status]=either&filters[original_id]=3003343&filters[translation_id]=45395719, warning was discarded
• 1. sync $ wp @translate wporg-translate sync-plugin-translations nothing-much/dev de --set=default
• 2. sync $ wp @translate wporg-translate sync-plugin-translations nothing-much/dev de --set=default
• Result: https://translate.wordpress.org/projects/wp-plugins/nothing-much/stable/de/default?filters[status]=either&filters[original_id]=3003393&sort[by]=translation_date_added&sort[how]=asc

This happens since #402. When a translation has warnings, GP sets the status to waiting but it doesn't check whether the same translation already exists with the same status.

cc: @akirk