I am using OSX so I don't know if this issue affects Windows.
Scenario:
Typing in a sketch, copy some code and paste it so I don't have to keep typing the same old stuff.
Problem:
Immediately after pasting, each time I go to a new line and start typing, I manage to get the first character down fine, then as soon as I type the second character, whatever was in memory for the previous copy/paste appears right where I'm typing. This occurs till I do the next copy/paste and then whatever is now in memory starts doing the same.
Question:
How do I stop this from happening? It's really giving me the Sh&%s
To the best of my reckoning, you are the first person to report this problem. It sounds like a rather nasty problem so, if the problem was widespread / common, I would expect every Arduino user running OSX to be pounding the forum with complaints. Which makes me suspect the problem is something specific to your computer.
Which version of Arduino are you using?
Have you ensured that you have the latest Java installed?
Could it be a display issue? Are the errant characters truly in the edit buffer or are they remnants on the display? Paging, scrolling, minimizing, maximizing, etcetera should reveal the truth.