When I go into the admin screen and click on the Manage Email Queue link, then from that screen click on Send Queued Campaign emails button, two emails go out for each queued email. Does anyone know what is causing this and how it can be prevented?

system:
Sugar: 4.5.1b (Build 1246)
Linux Kernel version 2.6.9-42.0.3.ELsmp
Apache version 1.3.34 (Unix)
PHP version 5.1.4
MySQL version 4.1.21-standard