This is not a bug.
I was listed only once initially.
When you click on the link that says "top x%" on any user's profile, it takes shows you the league and highlights the row of the respective user for you. You can see that the url contains the user id in this case. For example, if you click the "top x% on animuson's profile", it displays the league with his row highlighted and his userid appended to the url. http://stackexchange.com/leagues/4/year/meta/2014-01-01/141525#141525
This view is consistent irrespective of your user session (verify by logging out or using incognito mode). This shows the user only once.
Then I clicked to page #2 and then pressed the show 50 and I noticed I came up twice.
When you navigate in the league, the userid is no longer present in the url as a parameter. It identifies you from your user session.
The url is of the form: http://stackexchange.com/leagues/4/month/meta
View for logged in user:
View for anonymous user (no highlighted row):
If you are logged in, the top row always displays your stats, and is highlighted. The rest of the league is displayed below it. As a result, if you are #1, you can see yourself twice. For example, I see myself twice. If you aren't logged in (or in incognito mode), you will see the league without any repetitions.