I'm using symbols for a repeating image, and when I went in to adjust it found that it's copied itself four times. Every time I copy it and paste it behind something it creates a new symbol. Why and how do I stop that?
Also, is it possible to replace a symbol with another one, like in Flash?
This has not been "documented", it has only been reported. Nobody knows where it comes from, it's something in the file that goes corrupt. And it looks as if there are different reasons for it. Some of the files can be repaired using the methods described in the other thread, some can't.
Have you read the complete thread? You should note that you need to be very careful with files containing the problem, because it can be infectous.