title: Accuracy of adding float number to itself vs multiplication by 2
body: <p>Multiplying a <code>float</code>/<code>double</code> by 2 should be exact as it just increases exponent part.</p>
  <p>What about adding two floating numbers of same value? Will it be exact?</p>
  <p>(If it matters, hardware is standard x86-64).</p>