The /u/ vowel is spelled <oo>, <ue> and <o> in two. The /ʊə/ diphthong is spelled <our> or <ure>.
This is a contrast between a long vowel and a diphthong with similar starting positions. It is a known problem for Farsi and Swahili learners and probably for others, even some native speakers. Luckily there are few minimal pairs and so it does not matter very much.
The density figure is 1.8%. The set makes 10 semantic contrasts giving a loading of 52.6%.
allude allured boon bourn boons bourns boo boor boos boors cue cure cues cures endue endure endued endured endues endures moo moor mood moored moos moors queue cure queued cured queues cures too tour two tour twos tours
John Higgins, Shaftesbury, December 2009