Typically this error occurs because of one of the following reasons:
You have installed an older version of the software over the top of a newer version, and the two don't want to play nicely together (rare).
You have later installed an upgrade script with the same AppId as your original installer but without the same
[Code]
content (more common).You have used
[Files]
entries with wildcards to source files from a directory that already containsunins000.*
files (most common).
Refrain from doing those things and the error should go away. (In particular, for the last one: you should never use the installed location of your app as the source location for your installer. Always distribute "clean" files from a separate build/staging area.)