Strictly speaking, this is not a case of duplicate events in your calendar. Rather, you are looking at 2 different Google users' calendars who are attending the same event. Google helpfully layers the two calendars so you can see events from both at the same time.
This distinction is important as it explains why Google won't simply delete one of the events to make things more visually appealing.
If Account 1 is your primary Google account. The calendar for Account 2 has permissions set to share it with Account 1. Google won't remove events from Account 2's calendar simply because Account 1 is also an attendee (or vice versa).
So what can be done?
All that aside, your visual preference is not uncommon. Automatically modifying the calendars is a non-starter, however in the case of calendars being viewing via Google Chrome for desktops (as opposed to on Android/iPhone), there is an extension that will visually blend the events together. I have been using it for several years without any hiccups.
Event Merge for Google Calendar (chrome.google.com/webstore link) is a Chrome extension that visually merges the same event on multiple Google Calendars into one event.
It is also available on Github as Google Multi-Calendar Event Merge (imightbeamy/gcal-multical-event-merge) for Chrome, FireFox or Greasemonkey.