It has happened to me from time to time while I was typing a message to friend and sudden someone else sending a message which prompted for typing across each other
For some reason, other sender message straight to pop up on screen
It is like window in your house, wind is trying to get it so if you close all of it then wind can't get in
To clear pop up on your phone screen simple tab on navigation bar at bottom of your phone screen then select close all. It will shut down all applications that has been opened
