They could but they would run the risk of not getting a place at their chosen school.
It is interesting as a couple of years ago there was no mention of DC being prepped for the later tests and gaining advantage, now it seems rife. Tutoring seems to be reaching yet another level
Much as I hate the restrictions of free movement it causes maybe it is time for the Grammars to stick to one sitting a year and if you miss that then you have to wait for the following in year entry dates. Alternatively all late sittings recieve a different test, I know there is some debate about having enough DC to standardise fairly but with the number of authorities now converting to CEM surely this should be possible.