You still might have duplicate jar files. You might want to try this tool: https://github.com/cinaq/mendix-userlib-cleaner
See this example for double jar files
Also please make sure that you are using the appropriate version of any Marketplace modules. Often the versions of these modules change between major versions of Mendix. Check in the “releases” section of the marketplace for which version you should be using.
Thanks for your answers so far. I did use the cleaner tool and it was throwing just more errors. @Ronald Catersels: How would I recognize dublicates, e.g. what is the difference between the type Zulu Jar file and Required file?