I'm really late to the party, but I wanted to clarify what you experienced with the RSS feed, on why you could get old posts as new. The thing is proxigram grab the information from different sources (providers), not instagram.com directly (at least at the moment of writing this). So, let's say you are subscribing to @instagram feed, and the instance admin has set the maximum number of posts per feed, which is 12, proxigram makes a request to each of the twelve post to get the full information, so you are able to see the full content of the post directly from your reader.

The twelve request are being made at the same time and with each request a random provider is selected to scrape the necessary data, if an error occurs while fetching a post instead of pausing the process of generating the XML file, proxigram just ignore that post an returns what it was able to get (let's say 11 out of 12). Hours later, when you refresh the feed, the post that returned an error before, it didn't this time, but since your reader didn't have the id of that post at first, it treats it as a new post.

I could instead of this return an item saying that there was an error getting the post, and a link to the post. But since errors are pretty common, I think it would harm the experience of using RSS, but I don´t know. If people really want to add this, I could do it. What do you think?