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Appiah almost levelled the scores just moments later, but his long-range effort came crashing off the bar.
With four minutes of the first half remaining the hosts drew level as Appiah fired a strike into the top corner past Clarke.
Appiah then scored his second of the match on the stroke of half time when he saw his initial shot saved by the keeper, before chipping the rebound into the net.
Michael Raynes smashed an effort against the post in first-half stoppage time as Cambridge held onto their lead going into the break.