Marc Guehi stands on the brink of a rare double in the FA Cup . He has, however, already completed one. Few can testify more to the highs and lows the oldest competition of them all offers, the combination of ecstasy and ignominy. In consecutive appearances in the competition, the England defender helped inflict one of the greatest shocks in an FA Cup final and then suffered one of the biggest in any round. Guehi’s return to Wembley on Saturday is complicated, and not merely because he faces his former club Chelsea . A year ago, he was in the FA Cup final, as were Manchester City . But not together. Guehi captained Crystal Palace to the first major trophy in their 120-year history. Yet after the rise, came the fall. The holders were knocked out in January by sixth-tier Macclesfield , a non-league club 117 places below them. Guehi lifted Palace’s first major trophy, with victory against City in last season’s FA Cup final (Getty) And, in every previous campaign, that would have been Guehi’s FA Cup over.…