It is NOT a "Glitch" or "Bug", so to speak ...
In your case, it is NOT your "User Name" that you need to change, in fact, it is your "Contact Name" that that you need to change.
There are basically Four (x4) 'Different' Name formats, in 'FamilySearch', for Users/Patrons.
(1) Full ( Legal ) Name = Used to 'Create' a 'FamilySearch' Account;
..... usually, one's FULL 'Legal' name ... [ Always "Private", for your eyes only ]
(2) UserName = Used to "Sign In" ... [ Always "Private", for you eyes only ]
(3) Contact Name = Name that OTHER Users/Patrons see displayed for you...
..... [ Always "Public" ]
(4) Display Name = Name that is Displayed, on YOUR pages/screens,
..... for your 'FamilySearch' Account ... [ Always "Private", for your eyes only ]
That said ...
The "Contact Name" appearing TWICE is a particular 'quirk' of the "Community.FamilySearch" Forum, it is not a problem/issue in the other parts of 'FamilySearch'.
So, in your case what you need to "Change" is your "Contact Name".
That "Contact Name" for you can be "Changed" through YOUR "Settings", in your 'FamilySearch' Account (ie. NOT the "Community.FamilySearch" Forum).
Just Make sure there are two (x2) names, such as, a "Space" between Given and Surname; OR, one name with a "Space" and a 'Character' [any 'Character': eg. "." (ie. "Full Stop"/"Period")].
Otherwise you will end up with what you have now ... MaddieJohnson2 MaddieJohnson2 ... in the "Community.FamilySearch" Forum
In the "Contact Name", the reason for the two (x2) names and the space is that, this required for (Alphabetically) "Sorting" a User's/Patron's "Contact Name" within the "Community.FamilySearch" Forum - BUT, such it is NOT a requirement for the other Parts of 'FamilySearch'.
Remember: The "Community.FamilySearch" Forum is still an ongoing 'work in progress'; and, still evolving, every day.
I am do going to try and even debate why (lets not go there) ...
That is just what we have to do to work with the "FamilySearch.Communities" at the moment ...
ps: Here is a post from an "Official 'FamilySearch' Represntative" on the matter:
Mark MITCHELL (FamilySearch)
11 September 2020
I know many of you are frustrated that your username in the community is showing twice. First, I will tell you why this is and then I can tell you how to fix it. We found in the community that if we only had the names that were a single word put together then it would not be alphabetized properly because a single name ends up in the last name field. By showing that name as a first and last name, we were able to get it to alphabetize, but now we see the doubled names you are see in the system today. Here are the steps to get rid of this problem.
In a day or so the system will update your name in the community and your contact name will show as you type it.