Thanks for the query.
As both your blood test
ruled out the possibility of pregnancy after 10 days of missed period, the possibility of pregnancy is very less. Brown spotting
usually indicates collected blood in the endometrium.
And it is not the sign of pregnancy.
That may be indicating onset of your periods.
In that case no need to use duphaston
You watch for one to days, possibly by that time you will get your menstrual flow.
Delay in periods can occur due to many causes like hormonal imbalance, stress and strain, weight changes etc.
In the next cycle, you track your ovulation with ovulation kit and plan unprotected intercourse
around that period.
This will increase the possibility of pregnancy.