Multiple Tweets & Automatic Spam Filtering?

I've been testing Gowalla, which posts my location to my Twitter account as a tweet.

If I check-in to a location on Gowalla it posts a tweet with brief text and a short url to the location. By default the text sent to Twitter is exactly the same every time I check into a specific location.

I've noticed that after 4 days in a row of checking into my office and it posting to Twitter, that Twitter won't allow that tweet to be posted to my account anymore. It acts as if it is going to post, but is completely blocked and doesn't go through. Anything and everything else goes through fine, so I'm assuming it's some sort of automated spam feature recently implemented. If I copy and paste what would be posted and use the web, api, etc to post it, it won't go through, but if I change one character of the tweet it will.

Basically it seems to be a spam preventive measure on your end and I'd like to know if this is working as intended? If so can you give any details on how this is currently working? The tweets are about 24 hours apart, and aside from this everything else is working fine.

Should Gowalla use a different approach to this? For example, instead of posting the same short url every time for a location, should they be using a unique status update for the user at that location so all of their short urls are different?

Thanks