
You wouldn't look desperate sending another message, if you sent one this morning and no response either she didn't see it or isn't going to respond. If she doesn't respond after a second text later in the day then take the loss and go do something fun instead. Dating should be fun, no reason to play games and stress yourself out over a couple of messages.
If you texted already and she didn’t text back don’t double text to confirm plans, you’ll look desperate. Make her believe you can find something better to do. If she texts back so be it, have fun, if she doesn’t you dodged a bullet. If she felt you were important she would wake up looking for your text. Only excuse is if she was truly sleep.