Both S5's on my account have noticed messages will come through twice, but the first one will be half the original message and half of a text from weeks ago. the ornal message in its entirety comes in right after this mixed up message. Its very frustrating. Is there a fix for this??
Clear the app cache, and delete old messages you no longer need.
To clear the app cache, go to Settings > Apps or App Manager > find Messages+ and clear cache.