Using automator make an app that runs this applescript:

on run {input, parameters}
    set targetURL to "https://example.com" -- Replace with your desired URL

    tell application "Safari"
        activate
        set windowList to windows
        set windowFound to false

        repeat with currentWindow in windowList
            if (URL of current tab of currentWindow) contains targetURL then
                set index of currentWindow to 1 -- Bring the window to the front
                set windowFound to true
                exit repeat
            end if
        end repeat

        if not windowFound then
            -- No window with the URL found, create a new one
            make new document with properties {URL:targetURL}
        end if
    end tell

    return input
end run

Change the URL, and save it as an application, and add that application to the dock.

This will automatically detect a window you already have open with the URL and activate it instead.