The /ɔɪ/ diphthong is spelled <oi> and <oy>. The /ʊə/ diphthong is spelled <ure>, <oor> or <our>.
This is a contrast between two diphthongs with different starting positions and direction of movement. It is not a problem for learners.
The pair alloy/allure and its derivatrives is not completely minimal since the /ʊə/ element is usually palatalised.
alloy allure alloyed allured alloys allures boy boor boys boors buoy boor buoys boors doily dourly toy tour toyed toured toys tours