Actually, if you are using Maven3+, you'll get a warning like this:
[WARNING]
[WARNING] Some problems were encountered while building the effective model for groupId:artifactId:jar:1.0-SNAPSHOT
[WARNING] 'dependencyManagement.dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique: junit:junit:jar -> version 3.8.1 vs 3.0 @ line 15, column 18
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
Maven will pick up the first, so the second will never be used. Just remove it to prevent more confusion :)