I heard the same song 3 times in 2 days in shuffle mode

[ws1482]

I have a playlist with about 320 songs and it is a about 20 hours in length. I listen to my playlist for about  an hour or 2 everyday on shuffle mode. In the past two days I heard the same song 3 times. There are about 30 or 35 songs that play everytime and I barely hear the rest of the songs. We have been complaining about this for a long time. I am using an iPhone 4 with the latest version of the Spotify app