It has to be an exceedingly naive filter if it won&#x27;t catch &quot;which which&quot; as being highly likely to be an error and &quot;had had&quot; as being relatively common. False positives are of course possible [it&#x27;s always possible that you&#x27;re writing about writing errors], we&#x27;re talking about an aid though - I&#x27;d imagine a company like the BBC would use an in-house spell-checker of some form to check that documents match the style guide and that finding unlikely word duplications could easily be included.<p>How often is &quot;the the&quot; or &quot;which which&quot; actually intended, less than 1% of occurrences?<p>Buffalo.