Nope. I'll give the answer; you'll never get it.
It's: "Een gewaarschuwd mens telt voor twee". Freely translated it means: "A warned person is worth two". Obviously, as the man at the bridge is now twice himself, he can't cross the bridge. 😕🙄😕
The English equivalent of the proverb is "Forewarned is forearmed" but that doesn't work with this joke.