"imagent wants to use your confindential information" keeps popping up

Hi,

Recently, I've been getting a pop-up message saying "imagent wants to use your confidential information stored in "com.apple.facetime:registrationV1" in your keychain. Do you want to allow access to this item?"

I can click Always Allow, Deny and Allow, but none of these seem to have a real effect, as whatever I click, the message keeps coming back again. It seems to be linked to using iMessage, but I'm not sure. How can I stop this message from popping up every minute?