Double

C’est un double coquin = He is a thorough rascal.

C’est un homme double = He is a double-faced man.

Douceur

*Plus fait douceur que violence = Kindness does more than harshness; More flies are caught with honey than with vinegar.

[La Fontaine, Fables, vi. 3.]

Il faudra le prendre en douceur = You must tackle him gently.

Doute

Cela ne fait aucun doute = There is no doubt about it.

Dans le doute abstiens-toi = When in doubt, do nothing.