The woman's testimony is half compared to a man, ONLY in matters mentioned in Quran or hadees, eg financial matters as mentioned in Surah Baqarah..
In other cases, the testimony has equal weightage as that pf a man such as during SHAHADAH, or for LIAAN as mentioned in Surah Noor....
A woman does NOT need to repeat shahadah for two times, she just need only once to declare her faith...
When accusing husband, she does NOT need to say the words 8 times, while the husband has to say for 4 times...
Testimony is same in all matters, expect those of debt etc or financial matters.... here one could easily find out the reason when comparing the performance of men and women in maths...