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When dependencies are projects, building checks those projects in order to build and copy jars. When a project is a dependency of n projects you will see n attempts to build and copy this project. My proposal is not to make the attempt if the action was already done. Or collect the attempts before execution in a queue where duplicates are not allowed. A build with 21 projects will take 2 minutes and 46 seconds in a Satellite Pro A300, despite it had nothing to compile or copy.
This old bug may not be relevant anymore. If you can still reproduce it in 8.2 development builds please reopen this issue. Thanks for your cooperation, NetBeans IDE 8.2 Release Boss