Yes, launchd ensures you only have one instance of the "agent" running and it does this for you automatically. Set your job up normally using StartInterval to schedule the execution of your background job and the system won't launch a new instance if the previous instance hasn't completed by the time it gets to that scheduled start interval.

If multiple schedule intervals pass, launchd will run just one new instance at the next interval. It's "smart" about it.

Here's a sample plist:

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd  
"> 
<plist version="1.0"> 
<dict> 
     <key>Label</key> 
     <string>com.mycompany.test</string> 
     <key>ProgramArguments</key> 
     <array> 
         <string>/some/path/to/a/script.sh</string> 
     </array> 
     <key>StartInterval</key> 
     <integer>60</integer> 
</dict> 
</plist>

In the above example, if /some/path/to/a/script.sh takes 361 seconds to run, launchd won't start 5 new instances during that time frame, and it will only start one new instance at the 420'th second mark in time.

The wording on the launchd.plist(5) man page says as much, but the way it's written kind of makes it non-obvious:

StartInterval

This optional key causes the job to be started every N seconds. If the system is asleep, the job will be started the next time the computer wakes up. If multiple intervals transpire before the computer is woken, those events will be coalesced into one event upon wake from sleep.

That last sentence essentially says "launchd makes your jobs singleton for you as long as you follow the rules for good job behaviour".

To get a user LaunchAgent executing a script to run as root you have to do the following:

Modify the permissions of the script. Add the script (or command) to the sudoers file in the context of the user and finally add and load a proper launch agent plist.

This approach might create severe security holes!

In the example below I use the startup.sh of eXit-db 2.2:

• Create a plist:

  <?xml version="1.0" encoding="UTF-8"?>
  <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
  <plist version="1.0">
  <dict>
      <key>Disabled</key>
      <false/>
      <key>Label</key>
      <string>org.eXist_DB</string>
      <key>ProgramArguments</key>
      <array>
          <string>/usr/bin/sudo</string>
          <string>/Applications/eXist-db.app/Contents/Resources/eXist-db/bin/startup.sh</string>
      </array>
      <key>RunAtLoad</key>
      <true/>
      <key>StandardErrorPath</key>
      <string>/tmp/eXist_DB.err</string>
      <key>StandardOutPath</key>
      <string>/tmp/eXist_DB.out</string>
      <key>ThrottleInterval</key>
      <integer>10</integer>
  </dict>
  </plist>
  

  and save it as org.eXist_DB.plist in ~/Library/LaunchAgents

  The last key:

      <key>ThrottleInterval</key>
      <integer>10</integer>
  

  might not be necessary. In my VM it was - for unknown reasons.

• Modify the permissions of startup.sh:

  sudo chown root /Applications/eXist-db.app/Contents/Resources/eXist-db/bin/startup.sh
  sudo chmod 4755 /Applications/eXist-db.app/Contents/Resources/eXist-db/bin/startup.sh
  

• Modify the sudoers file:

  sudo visudo
  

• add a line to enable executing a command without the need to enter the sudo password

  # User privilege specification
  root    ALL=(ALL) ALL
  %admin  ALL=(ALL) ALL
  

  ->

  # User privilege specification
  root    ALL=(ALL) ALL
  %admin  ALL=(ALL) ALL
  your_user_name ALL=(ALL) NOPASSWD: /Applications/eXist-db.app/Contents/Resources/eXist-db/bin/startup.sh
  

• Finally enter:

  launchctl load -w ~/Library/LaunchAgents/org.eXist_DB.plist
  

  to load and start the launch agent