Delete the redundant User account using the Users&Group System Preference panel.
- Click the padlock icon, enter your admin-password, and click the "-" sign.
- Then pick one of the options "Save the home folder in a disk image" or "don't change home folder".
Both options will delete the acoount, but save the user data.
There is no "merge" command command for accounts; you can copy or move the files from the second accounts home-folder to a subfolder in your home folder.
You need to make yourself the owner of this second home folder.
And you need to be in an account that has Administrator Privileges for this.
- Open the Users-Folder in the Finder.
- Select the Home Folder to be moved.
- Select Get Info (File->Get Info or ⌘-I)
- In the Info window click the lock in the bottom right hand corner.
- Enter your password to open the lock.
- Click the "+" sign at the bottom left of the window.
- Select yourself in the popup
- In the gear pulldown select "make (me) owner"
- Click on the privileges next to your name, select read & write
- Select "Apply to enclosed items"
Now you should be able to move or copy all items to the corresponding folders in your Home folder.
If you are comfortable with the Terminal, you could change the ownership with the "chown" command instead of using "Get Info".