It appears that having Guard-Exit node in Tor increase the probability of deanonymizing the client compare to Guard and exit node separately. To put it in a nutshell if a node at times could become your Guard node and at other times could become your exit node even in a same session (look at this question for reference What is the risk of guard relay and exit relay being one and the same?), it might be able to integrate these information and thus facilitate client deanonymization. For further reference also please look at this question Why adding Guard-Exit (EE) node in Tor yeild more catch probability than adding guard and exit node separately?
Under these circumstances, why would Tor allow Guard-Exit node?