Ok, I found an old ragrepluginhook log but it is not about duplicate zones but if I remember correctly the reason for this error was an empty string value for one of the the xml element attributes, but I could not find any information in the log about which file or which xml attribute it was when looking in the log so I had to go through the entire file until I found the "".
Here is actually an example of a really helpful log that tells you even the line number: