Similar to prabhakaran's answer, in my case the problem was that I had created a form which had a LOT of logic in it (probably a symptom of bad design but there you go). To tame that complexity i had moved related sections of the code out into their own partial class files e.g. I had:
etc
Somehow Visual Studio generated .resx files for each of the partial class files as well as the primary file e.g. I had
etc
All of these partial class files all related to the same class 'MyForm', so all these .resx files all related to that same class, hence the message "The item was specified more than once in the "Resources" parameter."
All i had to do was delete all the extra .resx files leaving just 'MyForm.resx' and the problem was resolved.