Hi wehey,
That's a tricky question. I've only been able to come up with a half-solution. That is, the triggers will check the start time of each market and delete the ones that have identical start time with at least one other market. However, they don't give preferences, just blindly delete them leaving only one starting at that time.
Not sure if it'll help, try the attached file. Add the constants and two triggers to your own block of triggers. I assume that you start refreshing markets at X minutes before the start (i.e. not all at once).