Hi there. If the video you’re watching is longer than 5 mins, you may see 2 ads appear back to back. This should only happen for skippable ads or bumper ads (which are up to 6 seconds). Hope this clarifies.
It makes watching a 10-minute-video up to 30 minutes long if creators include 2 ad breaks. Very annoying. But I know you (and creators) only get money for ads and not for what viewers actually want you to do. But watching youtube recreationally is really hard that way.