Preventing Spammy User Accounts
It seems you have activated the enhanced recent changes where logs about operations (like creating user account) are grouped. The square bracket only contains the list of all created users for the same day. In the example you gave, Darren is separated because it's another day.
You can deactivate the enhanced recent changes in Special:Preferences#mw-prefsection-rc by unchecking the option and saving your preferences.
Thanks I was worried there for a moment. Is there anything else I can do to prevent spammers registering as the 2 who registered with a similar name sound like they are the same person.
I don't know about MediaWiki internal configuration for such limitation.
The user probably wanted to create a new account with better name, without knowing about account renaming.
If it's a small wiki and you know the people editing you could just ask him. Or ask on his talk page, but I agree: If this is a public wiki and ithere are no anti-spam measures set up, these two users might well be spammers.
To avoid spammers registering in your wiki, you can set up the Extension:ConfirmEdit, which comes with MediaWiki 1.18 and newer. It allows you to show your users a captcha when they register.