Isekai de Migawarihime Ninari Haou ni Ubawaremashita

When she slips and finds herself transported in another world, she is forced to take the place of a princess who looked exactly like her! The arrogant prince takes her captive and takes away her virginity! He's actually in love with her! Now will she return to her original world? A doting marriage with the most powerful prince?

(Source: Jewel Bunko, translated)