title: Qt - Clear old signals
body: <p>Say I have form A that can process signal x emitted by form B.
  When signal x was emitted, form A does something. Then I close form A, reopen form A and wait for signal x. However, the slot for processing signal x is called twice.
  I repeat closing and reopening form A. the slot for processing signal x is called an increasing number.</p>
  
  <p>It seems old signal still exits even though it was received.
  Is there any method to clear an old signal?</p>
  
  <p>Thank in advance!</p>