Dealing with mistake in scheduling an extra lecture
I don't really see how you could have done things that much differently: you made a mistake, tried to honor it so that nobody showed up and couldn't ask questions, and was open with everybody as much as possible. 

The only thing you might have done is ask people to attend one hour or the other, but they would have in principle every right to attend the full two-hour session if they so choose.

(That is, of course, in the event that you announced the wrong time. The easy way to fix this is to check, double-check, and triple-check the announcement before you post the announcement.)