I was seeing the same issues yesterday. For me it was happening when my phone was unlocked and I was just on the springboard. I also have intelliscreenX installed. I was not getting the double tone when my phone was locked with intelliscreen receiving the texts. After I messed around with selected the alert tone I wanted and a respring, the error went away. I am waiting to see if it comes back on its on like some users are experiencing. I hope its just a quick respring fix and be done with it.
Exact same thing here. But it only happens to me when I get a text with using ISX on the Springboard. Kind of annoying... But I can live with it.