It's a very common question, and a very common practice.

Many endos will start you on oestrogen only initially, as it can already drive down your T-levels on its own. After 6-12 weeks (depending on endo and next bloodwork appointment), they may then decide, based on your T-levels, to add an anti-androgen. Ideally you won't need it, as your body just reacted THAT well to the oestrogen, but it's no issue if you do either. The less pills you have to take the better though.