To be picky, the issue here isn't really two instances of the rule "running" at the same time, but there would indeed likely be a problem with your rule as written above. (Hubitat apps, including rules, aren't always "running," just waiting for a scheduled event, subscription, or something else to wake them up if they aren't actually doing something at that moment. In the case of a delay, a rule makes a scheduled event for that far into the future that wakes it up and resumes operation. In the meantime, it isn't really "running." There are rare issues with two instances of a rule truly running at the same time, which can cause problems in that they share a single state and one could overwrite the "other's" [but again, actually the same] with unexpected data--consider issues people had a while back with "contact changed"-type triggers and contact sensors that sent duplicate "open" events within milliseconds of each other. But I digress.)
The issue here is that the "cancelable" flag that you (I'd say correctly) set on the "Delay" action won't do anything on its own. This just marks it as being eligible for cancellation. A "Cancel Delayed Actions" action in this rule would actually stop it (while ignoring delays without this set). If you're familiar with webCoRE on ST (or Hubitat, I guess), this is similar in concept to its Task Cancellation Policy (TCP) feature, except that TCP is on by default, while RM makes you create the same effect manually with the interaction of these two things. (Of note, a "Cancel Rule Timers" action can be applied to one rule from another, and this will stop any delay, with or without "cancel?" selected, but it also does a bit more and might be overkill for most situations.)
If this still isn't clear, the Rule 4.0 docs have a bunch of examples with cancellation, many in the context of motion lighting where the desirable and undesirable outcomes are easy to understand (e.g., you want the delay to get cancelled if motion becomes active again). I'm not exactly sure how you'd factor something like that into your garage door rule (what do you want to "cancel" this with? would a virtual [or even real] switch you could turn on when you want to override the rule be a better choice?), but I'm sure you have some ideas.
