What is the meaning of the encounter between Duessa and the Redcrosse Knight on stanzas 44-51? How do the descriptions reflect this meaning?
In this episode, Redcrosse is sent a dream of Duessa, who appears in the likeness of Una. Unlike Una, however, Duessa is schooled in the arts of illusion, and seeks with 'false shewes [to] abuse his fantasy.' Giving him a vision of 'lustfull play', she repines that Cupid has 'her chast hart..subdewd, to learn Dame Pleasures toy.'
Caught between lust and moral outrage, Redcrosse starts awake, beholding then the vision of his dream (you could compare with 'Eve of St Agnes'). In his anger, he nearly slays Duessa/Una outright, but stops himself, hoping to 'test' his lady by using the proof of his senses.
The episode is psychologically and allegorically complex. Spenser makes it clear that Redcrosse, who thinks he is able to see falsity in Una, is himself being tested, and failing. As soon as he abandons faith to rely on the evidence of his senses, he is lost, at least temporarily. Spenser reflects Duessa's duplicity through jangling, convoluted and alliterative diction, ('do, or die./Die is my dew: yet rew my wretched state'), the internal rhyme echoing the moral mazes into which she leads him. He is not free of the power of illusion and false faith until much later in the book, and requires Arthur's help to achieve it.