Hi Yáng!
Thanks for your comment, I'm glad you liked the game!
Yes, Duran is one of the toughest one to romance (but not to get laid with).
If you are not able to spend the evening 14 with him after talking to Rania before, then it means that your relationship with him is too weak... The most likely reason is that you let a lot of mercenaries die (he really doesn't like when you are not able to save them).
But here is a shortcut: he will really love the leather bracelet you can buy in Halmont's jewelry shop (he will also like the amulet of protection but a bit less).
I hope this helps ^^
Otherwise, a detailed walkthrough is available for ko-fi members ;-).
Cheers,
Azephir