AppleScript to find existing tab with URL, or open a new one.

safari_find_or_open_url.scpt

tell application "Safari"
	set topWindows to every window whose name is not ""
	set numWindows to the number of topWindows
	set didFind to false
	set targetUrl to "http://localhost:3000/"
	
	repeat with x from 1 to numWindows
		set numTabs to the number of tabs in window x
		repeat with y from 1 to numTabs
			set tabUrl to the URL of tab y of window x
			if tabUrl contains targetUrl then
				set didFind to true
				tell window x to set current tab to tab y
			end if
		end repeat
	end repeat
	
	if didFind is false then
		tell window 1 to set current tab to (make new tab with properties {URL:targetUrl})
	end if
end tell