As ocd as I am with this phone, that can't be it. I wipe the screen atleast once a day with a cleaning cloth and it's always the same button.
If you slide out the keyboard at the home screen and press any direction on the d pad, you'll see that this is a selection cursor for selecting apps or widgets with only the keyboard. Probably a holdover from devices with a trackball. I've noticed that it only happens when texting with the hard keyboard, then going home and closing the KB in rapid succession.
I couldn't tell you how to fix it other than to text using the soft keyboard only...
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I have a similar texting problem. When I try to use the phones keyboard (not touch screen) some letters I push show up 2 or three times even though I only push them once. No big deal but maybe there is a fix. JW