A pair of BFFs in Book 4, The Book of Friendship, Amyas and Placidas manifest their closeness through their indistinguishable appearances. Apparently, these two friends look so alike not even a love interest, Poeana, can tell them apart.
This proves quite handy when Amyas is captured by Poeana and almost forced to be unfaithful to his real love, Aemylia. Lucky for him, his friend Placidas, who is unattached, steps in and happily romances the princess, ultimately marrying her and becoming king. While making these two friends look like identical twins might sound extreme, it was a common belief in the Renaissance that true friends were almost two versions of the same person. So here, Spenser is just quite literally illustrating that.