I just had unprotected sex and im taking duphastone pills for my irregular period, can i take emergency contraception pills like plan b to prevent pregnancy?

"Plan B" can be taken with the information provided, but chances are that ovulation did not occur given your history of irregular periods and the "duphastone" regimen. "Plan B", in any case is not 100% certain to prevent pregnancy, so in 20 days, get a "home" pregnancy test for reassurance. See your GYN physician for guidance on taking regular oral low dose bcp's.