Duplicate object fix.
Duplicate object fix.

Summary: Switched from cerr to atNotifier. Added a check to make sure duplicate actors are not added to the scene.

Test Plan: Tested in opensim standalone with default region on 64 bit fedora.

Reviewers: ssirigam, rlouden, martin, kboswell

Reviewed By: ssirigam, rlouden, martin, kboswell

Differential Revision: http://cr.irl.ucf.edu/D367

ucfistirl/physx-wrapper,ucfistirl/physx-wrapper