assurance doubly sure
From Shakespeare's Macbeth, Act IV, Scene i:
Macbeth! Macbeth! Macbeth!--
Had I three ears, I'ld hear thee.
Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth. [Descends.]
Then live, Macduff: what need I fear of thee?
But yet I'll make assurance double sure,
And take a bond of fate: thou shalt not live;
That I may tell pale-hearted fear it lies,
And sleep in spite of thunder.