I’m seeing this issue frequently.
I’m using the API to post a tweet (using twitteroauth PHP library). Sometimes, everything works fine (I get a 200 HTTP response, and can read the tweet ID from the response.
However, I often get at non-200 HTTP response, and error code 187, saying the tweet is a duplicate.
The problem is that a) there definitely isn’t a duplicate tweet on the user’s timeline, and b) the tweet still gets posted.
This is a problem for me because I can’t get the tweet ID, so can’t look up any stats for that tweet for the user. It also means I can’t reliably tell whether a tweet has been posted or not and handle it appropriately.