Yes, this. Because Ianto Jones was dead and this was a clone in its place. Jack had dealt with the clones, and, frankly, was sick and tired of them. He didn't want a threesome (or a foursome) -- and he wanted nothing more than to try and sleep.
Ianto shouldn't see Jack's face because said face was a masque of hatred and anger.
"No," Jack said simply, eyes narrowed. "Because you clones saw this, knew this is how I can tell him apart from the others. So I don't care if you went through his mind to find out how to snog me before you killed him. I don't do 'second bests'." Jack waved his hand, wishing he had his webley to shoot the clone between the eyes. "So get the fuck out of here and go take your failure back to the Powers. And, while you're at it, you can fucking tell them to go to hell and that I'll be happy to send them on the trip."