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.