If a figure has both ward and brittle and suffers damage, the two conditions negate each other and are both removed, with neither effect applying to the damage.

This is stated on page 28:

• "If a figure that suffers damage has both ward and brittle (see p. 29), they negate each other and are removed."

So, when damage is suffered:

• Ward and brittle are both removed.
• The damage is resolved as normal, with neither ward (halving) nor brittle (doubling) affecting the amount.

Source: page 28.

Source: Core Rulebook (pp. 27, 28, 29, 38, 76)