Some final touches for the Issue Relationships (masc)

Main change is that while resolving an issue, if the user enters a duplicate id (while the relationship already exists), the system will not generate an error anymore.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2706 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9