We treat the number as accurate as when an email is opened at least once it gets counted. If it’s open more than once it doesn’t get counted that way.
However, it’s pretty easy to test. Send an email to 5 people and have all five people open it once. You should see the open rate at 100%. Then have each of those five people open it a second time. Their contact record should show they have opened more than once but your open rate should still be 100%.