A faint second line on a home test usually still counts as positive, since even a little hCG shows up. Your light spotting lasted only fifteen minutes and wasn't like your usual full flow. That pattern fits an early pregnancy more than a regular period.
Next Steps
Repeat the test after two to three days using your first morning urine, or ask your doctor for a blood beta-hCG test for a faster answer.
Health Tips
Please tell your doctor about this result, since whether you continue or stop the Duphaston can depend on whether it's a pregnancy.