In some LAN configurations, there may be more than one Mac used to manage the others using Apple Remote Desktop. These may be physically separated by some distance. In such a situation, you may need to take control of one of the managing Macs from the other. But Remote Desktop won't let you control or view another Mac that is already running Remote Desktop.

If you can ssh to that Mac, you can manually quit the program from the command line. If that Mac's settings allow other VNC clients to connect to it, you can also do that. But if those options aren't available, here's a simple trick to quit the remote Remote Desktop. Create an AppleScript with the following code:

tell application "Remote Desktop"
  quit
end tell

Save it as an application, and test it locally. Then select the remote Mac from ARD's clients list and then Manage » Open Application. After ARD has grabbed the list of remote applications to pick from, click the + icon below the list and select your local AppleScript application.

The script is sent to the remote Mac and executed, quitting Remote Desktop there. You can now take control of that Mac as normal.