I dep fairly regular and as a result I'd have taken the path of least resistance and do the single remaining date... so long as you haven't turned other work down
However, I'd also make note (I use a Google Calendar) and if they do something similar in the future, I'd politely point out that you are setting aside dates/time for them (could be that you had wanted to do something socially) but this seems to be recurring and in future you'll not be able to help. Tis is of course assuming they aren't just mates!
I've had to do this with a band as I've gone out of my way to learn material only to be told I'm not needed (generally bassist plans change and can now do gigs)... twice.