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It happens that multiply libraries/jars that are attached to a project classpath contain class files with the same package and class names (duplicates). The order in which these appear in the list determines which of the duplicates are used and which are ignored. The developer MAY be aware of duplicates and may control them by ordering classpath entries, but usually duplicates are not recognized and act like dangerous traps. I am aware of a native compiler that warns of duplicates (JET) and there may be others. It would be very valuable if the IDE could warn the developer by default of such duplicates in the build process. It could be argued that this could be carried out by an optional process such as validation, but I would prefer the check to be always on, mandatory, or optional and on by default.
Would it be possible to evaluate this? I just discovered that it is not possible to deploy duplicate classes on Android, while the Eclipse IDE does not emit any warnings before it is too late.
The check will be probably better in time when you add a jar with duplicate on CP, not to slow down the build. The IDE will need to list at least packages of all jars and if there are the same packages in more jars it needs to list files in these packages.