Digital certificates in Outlook 2007

Hello,

I've been using Outlook 2007 for a while with a digital certificate I have
from Comodo. It is annoying how every signed e-mail I sent, Outlook has to
ask me _twice_ about granting access to the certificate in order to send
_any_ e-mail I want to sign.

I have already trusted the certificate using the Trust Center in Outlook but
it doesn't help. If the window for granting/denying access showed up only
once I would not be bothered at all, but why twice? I didn't have this
problem in Outlook Express nor in Outlook 2003.

Is there anyway to fix this? I'd be very thankful.

Thanks,
Rebêlo.