Everyone is right on target, it won't hurt your horse to deworm again that soon, but it's likely that you won't have many (if any parasites) that will be killed by deworming again. When exactly in July are they going to be deworming? It's probably best just to go ahead and do it so that everyone is happy.
Kansas-twister, you said you use Safe-guard....are you having regular FEC's done? Fenbendazole has extremely high incidence of parasite resistance (95% of farms tested, and the situation is world-wide) so it's very likely that you aren't getting a good kill by using just that product. It would be a great idea for you to monitor efficacy by doing an FEC just before deworming and then again 2 weeks later to see how much of a change there is in the number of parasite eggs being shed.
As for rotating products, that's a whole bunch of speculation and there is nothing to prove that rotating slows parasite resistance at all. The practice of rotating dewormers actually stems from the need to rotated the dewormers that were used many years ago because the ones that were available only killed one or 2 types of parasites and so you needed to used different chemicals each time to be sure you killed the different types of parasites that are a problem. The biggest key to preventing resistance is actually ensuring that you are not under-dosing. Giving a smaller than recommended dose of dewormer means that you will have more parasites that get exsposed to the chemical but not killed by it and thus can build up resistance and pass it on to their progeny.
Licensed Veterinary Technician