Though I've seen something like this in production once before, I can only replicate this on dev after the fix for bug 906058 went in. If a snippet is available in multiple locales, and a user's client matches more than one of those locales, the snippet will appear in their snippet results multiple times. For example, if a snippet is available in `es` and `es-MX`, users in `es-MX` will get it twice in their snippet results, causing it to show up more often than other snippets.
Commit pushed to master at https://github.com/mozilla/snippets-service https://github.com/mozilla/snippets-service/commit/d6da1191faf6b41c32bde4883006ddd2dd579aa8 Fix bug 909817: Ensure snippets from match_snippets are distinct.
Verified that clients matching multiple locales for a single snippet are now only receiving one copy of that snippet.