How to open the existing file explorer and not a new one
You may have the "Open each folder in its own window" check box selected in the Behavior tab of File Management Preferences.
Select a folder From the top menu bar, select Edit/Preferences Behavior tab Uncheck "Open each folder in its own window"
It would help to know which version of Ubuntu you are using; this answer is a guess. Edit your question with the output of this command from the terminal:
lsb_release -a