brelausto, did this fix your problem ?
I also have this problem and can't seem to fix it.
I've done factory reset and reloaded the software using the pc utility but the problem remains.
I have discovered what triggers it - If 4G is enable it seems texts will get sent more than once. If 4G is disabled it works fine.
This applies even when my data connection is turned off.
In another test I get the following results when sending a message to myself
4G off - I receive the text immediately
4G on - I received the text approx 15-20 seconds after I send it and then I receive it again some time later. Sometimes I get it 3 or 4 times
I assume this is something to do with the phone switching between 4G and whatever else it uses when sending texts ?
It's only recently started doing this.
Any ideas much appreciated.